Tweak error message when external image editor launching fails.

Include idea that Blender may fail to launch it even if path is correct,
in some cases (dear Windows...).

Based on idea from @lijenstina and @blendify (D2349), thanks.

release/scripts/startup/bl_operators/image.py

@@ -82,8 +82,8 @@ class EditExternally(Operator):
             import traceback
             traceback.print_exc()
             self.report({'ERROR'},
-                        "Image editor not found, "
-                        "please specify in User Preferences > File")
+                        "Image editor could not be launched, please ensure that "
+                        "the path in User Preferences > File is valid, and Blender has rights to launch it")
 
             return {'CANCELLED'}