...of type 'No such file or directory' to the CPlusPlus::Document. Only
the first one was added and as a result, the editor only wavely
underlined that one.

Add also diagnostic messages if it's not possible to get the file
contents.

Change-Id: I8389d8e6af9480ea6712759ce5e130e1dd8912f1
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>