Sanitize error/finished handling in executor jobs and command executors.

The classes as they are now specify different signals for signaling
failure and finishing. Some of these error() signals are followed by a
finished() signal, others are not, which is just asking for trouble.
Under certain circumstances there can also be several error() signals
for one and the same process. In fact, there is almost certainly a race
condition there regarding executor job re-use.
This patch changes the design so that there is always exactly one
finished() signal that carries an error status. For some non-fatal
problems that higher-level code cannot sensibly handle, we now log a
warning instead of emitting an error signal.

Change-Id: I9e3df11564e7337ad766ca0d009303367d43c4ec
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
fix module property cache

When caching module properties, we must take into account whether
we're requesting one or multiple values. Otherwise we might get
the wrong cached result.

Task-number: QBS-539

Change-Id: I9ca17d3a537922f37dc70be4dbfd73416b6bdfc3
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>