Small change to job failure output

Currently, job failures are done by raising OpExecError(job result). For
a one-opcode job that failed, this is very non-intuitive:
Failure: command execution error:
[u'Disk size change not possible, use grow-disk']

This patch changes the output in two ways:
 - for a job which didn't have opcodes which succeeded, it changes it
   to:
   Failure: command execution error:
   Disk size change not possible, use grow-disk
 - for a job with partial completion, to:
   Failure: command execution error:
   partial failure (opcode 1): Cannot remove the last disk of an
   instance

The actual error type is not changed. A job that doesn't have at least
an error-ed opcode is shown as before.

Reviewed-by: imsnah