Handle missing ctags more gracefully in currenttag

If tagbar#currenttag() was used in a statusline on a system that had
Tagbar installed, but not ctags, then an error was reported on every
statusline update. Change the ctags checking so that errors in
currenttag() fail silently while still reporting an error message if
Tagbar is opened normally.

autoload/tagbar.vim

@@ -83,6 +83,7 @@ let s:icon_open   = g:tagbar_iconchars[1]
 
 let s:type_init_done      = 0
 let s:autocommands_done   = 0
+" 0: not checked yet; 1: checked and found; 2: checked and not found
 let s:checked_ctags       = 0
 let s:checked_ctags_types = 0
 let s:ctags_types         = {}
@@ -102,9 +103,11 @@ let s:debug = 0
 let s:debug_file = ''
 
 " s:Init() {{{2
-function! s:Init()
-    if !s:checked_ctags
-        if !s:CheckForExCtags()
+function! s:Init(silent)
+    if s:checked_ctags == 2 && a:silent
+        return 0
+    elseif s:checked_ctags != 1
+        if !s:CheckForExCtags(a:silent)
             return 0
         endif
     endif
@@ -921,7 +924,7 @@ function! s:RestoreSession()
 
     let s:last_autofocus = 0
 
-    call s:Init()
+    call s:Init(0)
 
     call s:InitWindow(g:tagbar_autoclose)
 
@@ -1004,41 +1007,48 @@ endfunction
 " s:CheckForExCtags() {{{2
 " Test whether the ctags binary is actually Exuberant Ctags and not GNU ctags
 " (or something else)
-function! s:CheckForExCtags()
+function! s:CheckForExCtags(silent)
     call s:LogDebugMessage('Checking for Exuberant Ctags')
 
     let ctags_cmd = s:EscapeCtagsCmd(g:tagbar_ctags_bin, '--version')
     if ctags_cmd == ''
-        return
+        let s:checked_ctags = 2
+        return 0
     endif
 
     let ctags_output = s:ExecuteCtags(ctags_cmd)
 
     if v:shell_error || ctags_output !~# 'Exuberant Ctags'
-        echoerr 'Tagbar: Ctags doesn''t seem to be Exuberant Ctags!'
-        echomsg 'GNU ctags will NOT WORK.'
-              \ 'Please download Exuberant Ctags from ctags.sourceforge.net'
-              \ 'and install it in a directory in your $PATH'
-              \ 'or set g:tagbar_ctags_bin.'
-        echomsg 'Executed command: "' . ctags_cmd . '"'
-        if !empty(ctags_output)
-            echomsg 'Command output:'
-            for line in split(ctags_output, '\n')
-                echomsg line
-            endfor
+        if !a:silent
+            echoerr 'Tagbar: Ctags doesn''t seem to be Exuberant Ctags!'
+            echomsg 'GNU ctags will NOT WORK.'
+                  \ 'Please download Exuberant Ctags from ctags.sourceforge.net'
+                  \ 'and install it in a directory in your $PATH'
+                  \ 'or set g:tagbar_ctags_bin.'
+            echomsg 'Executed command: "' . ctags_cmd . '"'
+            if !empty(ctags_output)
+                echomsg 'Command output:'
+                for line in split(ctags_output, '\n')
+                    echomsg line
+                endfor
+            endif
         endif
+        let s:checked_ctags = 2
         return 0
     elseif !s:CheckExCtagsVersion(ctags_output)
-        echoerr 'Tagbar: Exuberant Ctags is too old!'
-        echomsg 'You need at least version 5.5 for Tagbar to work.'
-              \ 'Please download a newer version from ctags.sourceforge.net.'
-        echomsg 'Executed command: "' . ctags_cmd . '"'
-        if !empty(ctags_output)
-            echomsg 'Command output:'
-            for line in split(ctags_output, '\n')
-                echomsg line
-            endfor
+        if !a:silent
+            echoerr 'Tagbar: Exuberant Ctags is too old!'
+            echomsg 'You need at least version 5.5 for Tagbar to work.'
+                \ 'Please download a newer version from ctags.sourceforge.net.'
+            echomsg 'Executed command: "' . ctags_cmd . '"'
+            if !empty(ctags_output)
+                echomsg 'Command output:'
+                for line in split(ctags_output, '\n')
+                    echomsg line
+                endfor
+            endif
         endif
+        let s:checked_ctags = 2
         return 0
     else
         let s:checked_ctags = 1
@@ -1590,7 +1600,7 @@ function! s:OpenWindow(flags)
     " This is only needed for the CorrectFocusOnStartup() function
     let s:last_autofocus = autofocus
 
-    if !s:Init()
+    if !s:Init(0)
         return 0
     endif
 
@@ -3374,7 +3384,7 @@ function! tagbar#autoopen(...)
     call s:LogDebugMessage('tagbar#autoopen called on ' . bufname('%'))
     let always = a:0 > 0 ? a:1 : 1
 
-    call s:Init()
+    call s:Init(0)
 
     for bufnr in range(1, bufnr('$'))
         if buflisted(bufnr) && (always || bufwinnr(bufnr) != -1)
@@ -3403,8 +3413,8 @@ function! tagbar#currenttag(fmt, default, ...)
         let fullpath = 0
     endif
 
-    if !s:Init()
-        return ''
+    if !s:Init(1)
+        return a:default
     endif
 
     let tag = s:GetNearbyTag(0)
@@ -3418,7 +3428,7 @@ endfunction
 
 " tagbar#gettypeconfig() {{{2
 function! tagbar#gettypeconfig(type)
-    if !s:Init()
+    if !s:Init(1)
         return ''
     endif