added syntax warning for incorrect snippet files

syntax/snippet.vim

@@ -7,6 +7,7 @@ syn match snipCommand '`.\{-}`'
 syn match snippet '^snippet.*' transparent contains=multiSnipText,snipKeyword
 syn match multiSnipText '\w\+ \zs.*' contained
 syn match snipKeyword '^snippet'me=s+8 contained
+syn match snipError "^[^#s\t].*$"
 
 hi link snipComment   Comment
 hi link multiSnipText String
@@ -15,3 +16,4 @@ hi link snipComment   Comment
 hi link placeHolder   Special
 hi link tabStop       Special
 hi link snipCommand   String
+hi link snipError     Error