Wrap saveScreenState's statements in a try-catch block. (#1047)

* Wrap saveScreenState's statements in a try-catch block.

If you're in NERDTree, and you press : to open a command line, and then
press Ctrl+F to open the [Command Line] window, you will see errors
related to switching windows while in this function. (Those commands are
not allowed while in the [Command Line] window.) The try-catch block
handles the errors, preventing their being displayed.

* Update version number in change log.

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Change Log
 
 #### 6.1...
+- **.2**: Wrap saveScreenState's statements in a try-catch block. (PhilRunninger) [#1047](https://github.com/scrooloose/nerdtree/pull/1047)
+- **.1**: Catch errors when trying to read CHANGELOG.md. (PhilRunninger) [#1045](https://github.com/scrooloose/nerdtree/pull/1045)
 - **.0**: If file path doesn't exist, :NERDTreeFind its parent directory instead. (PhilRunninger) [#1043](https://github.com/scrooloose/nerdtree/pull/1043)
 #### 6.0...
 - **.1**: Reintroduce necessary variable mistakenly removed. (PhilRunninger) [#1040](https://github.com/scrooloose/nerdtree/pull/1040)

autoload/nerdtree.vim

@@ -10,8 +10,9 @@ let s:rootNERDTreePath = resolve(expand("<sfile>:p:h:h"))
 "  change log is shown for the current version; otherwise, only the version
 "  number is shown.
 function! nerdtree#version(...)
-    let l:changelog = readfile(join([s:rootNERDTreePath, "CHANGELOG.md"], nerdtree#slash()))
     let l:text = 'Unknown'
+    try
+        let l:changelog = readfile(join([s:rootNERDTreePath, "CHANGELOG.md"], nerdtree#slash()))
         let l:line = 0
         while l:line <= len(l:changelog)
             if l:changelog[l:line] =~ '\d\+\.\d\+'
@@ -21,6 +22,8 @@ function! nerdtree#version(...)
             endif
             let l:line += 1
         endwhile
+    catch
+    endtry
     return l:text
 endfunction
 

lib/nerdtree/ui.vim

@@ -355,12 +355,15 @@ endfunction
 " scroll position
 function! s:UI.saveScreenState()
     let win = winnr()
-    call g:NERDTree.CursorToTreeWin()
     let self._screenState = {}
+    try
+        call g:NERDTree.CursorToTreeWin()
         let self._screenState['oldPos'] = getpos(".")
         let self._screenState['oldTopLine'] = line("w0")
         let self._screenState['oldWindowSize']= winwidth("")
         call nerdtree#exec(win . "wincmd w", 1)
+    catch
+    endtry
 endfunction
 
 " FUNCTION: s:UI.setShowHidden(val) {{{1