Rewrite switch on wxTextBufferOpenMode to be exhaustive

Get rid of the default clause to allow the compiler to warn us if ever add new
elements to this enum.

No real changes.

src/common/textfile.cpp

@@ -58,14 +58,10 @@ bool wxTextFile::OnExists() const
 
 bool wxTextFile::OnOpen(const wxString &strBufferName, wxTextBufferOpenMode openMode)
 {
-    wxFile::OpenMode fileOpenMode;
+    wxFile::OpenMode fileOpenMode = wxFile::read_write;
 
     switch ( openMode )
     {
-        default:
-            wxFAIL_MSG( wxT("unknown open mode in wxTextFile::Open") );
-            wxFALLTHROUGH;
-
         case ReadAccess:
             fileOpenMode = wxFile::read;
             break;
@@ -75,6 +71,13 @@ bool wxTextFile::OnOpen(const wxString &strBufferName, wxTextBufferOpenMode open
             break;
     }
 
+    if ( fileOpenMode == wxFile::read_write )
+    {
+        // This must mean it hasn't been initialized in the switch above.
+        wxFAIL_MSG( wxT("unknown open mode in wxTextFile::Open") );
+        return false;
+    }
+
     return m_file.Open(strBufferName, fileOpenMode);
 }