Core: Let IDocument also report failed file reload operations

Change-Id: I8fa02f2398f59306c49481316fdedf50a1c4e32f
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
CppTools: Do not hand over partly reloaded files to the code model

When a reloading operation was in progress, CppEditorSupport::contents()
could return a partly reloaded file to the code model leading to parse
errors and incomplete highlighting.

Fixed by checking if the file is currently being reloaded.

Task-number: QTCREATORBUG-9382

Change-Id: Iee97e10444763c0cbf481132afa4617c5cdbd15c
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>