IdentityRuntime: expand the process error warning

I have encountered the following warning in KDevelop's output:
kdevplatform.shell: process finished with error: QProcess::Crashed

There is no way to tell which process crashed, especially if the issue
cannot be reproduced. Print the error string and the command line of the
process to specify where the error comes from. For example:
kdevplatform.shell: process finished with error: QProcess::Crashed "Process crashed", the command line: "make -j5"

Introduce a helper function catchErrors(). Cannot simply delegate the
KProcess* overload to the QProcess* one, because KProcess::start() hides
non-virtual QProcess::start().