call_and_check does not report error output

[Jeroen T. Vermeulen]

We use call_and_check in several places, but we never make any special provisions for its error output. In tests on a small cloud instance I'm getting out-of-memory and out-of-disk-space errors while importing images, and the exceptions are logged, but the reasons for the exceptions are not. This makes the problems very hard to diagnose.

For better debugging, call_and_check ought to capture the command's stderr by default and, in the event of a CalledProcessError, set it as the exception's "output" instance variable.