CppTools: Add all diagnostic messages

...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>
CppTools: Add diagnostic message for non-existent absolute paths

Change-Id: I9a05a92c1adc37db0c918127715714d1f6d2a78d
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>