Show complete command with args when ComputePatch fails

When Difference.ComputePatch() failed for some reason, only
the first part of the failed command was show with the error,
not all arguments.

old (simplified):
WARNING: failure running ['imgdiff', '-b', '/tmp/recovery-resource.dat']:

new:
WARNING: failure running ['imgdiff', '-b', '/tmp/recovery-resource.dat',
  '/tmp/tmpSsy_kf', '/tmp/tmpOMCXSn', '/tmp/tmpA3FtZw']:

Change-Id: Id8b498df3ed7934815430e4b4c2651ef4aa781c6