Move some more message strings to the properties file.

common/locale/en-US/messages.properties

@@ -100,6 +100,12 @@ io.exists-1 = File %S exists (add ! to override)
 io.noCommand-1 = Command not found: %S
 io.commandFailed = E472: Command failed
 io.oneFileAllowed = E172: Only one file name allowed
+io.callingShell-1 = Calling shell to execute: %S
+io.sourcing-1 = sourcing %S
+io.sourcingEnd-1 = finished sourcing %S
+io.notInRTP-1 = not found in 'runtimepath': %S
+io.searchingFor-1 = Searching for %S
+io.searchingFor-2 = Searching for %S in %S
 
 macro.canceled-1 = Canceled playback of macro '%S'
 macro.recorded-1 = Recorded macro '%S'
@@ -123,6 +129,7 @@ mow.noPreviousOutput = No previous command output
 
 option.noSuch = No such option
 option.noSuch-1 = No such option: %S
+option.replaceExisting = Warning: %S already exists: replacing existing option.
 
 plugin.searchingFor-1 = Searching for %S
 plugin.searchingForIn-2 = Searching for %S in %S