Handle wrong rtp more gracefully

If the runtimepath has been set incorrectly and the autoload file is
getting loaded by something, then the code won't be able to load the
"plugin" file. Print a warning message and stop loading the file instead
of throwing lots of errors.

autoload/tagbar.vim

@@ -28,6 +28,11 @@ if exists(':Tagbar') == 0
     runtime plugin/tagbar.vim
 endif
 
+if exists(':Tagbar') == 0
+    echomsg 'Tagbar: Could not load plugin code, check your runtimepath!'
+    finish
+endif
+
 " Basic init {{{2
 
 redir => s:ftype_out