get rid of stacktrace when pbuilder fails

Errors like the one below are very confusing. The first thing you usually see is a python/pbuilderjenkins stacktrace which makes you immediately think its a pbuilderjenkins error. Only if you have a closer look you realize its a legitimate build failure.

Please catch the exception and log an error message instead.

make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2
E: Failed autobuilding of package
W: no hooks of type C found -- ignoring
I: unmounting results filesystem
I: unmounting /var/cache/pbuilder/ccache filesystem
I: unmounting /var/cache/ccache filesystem
I: unmounting dev/pts filesystem
I: unmounting proc filesystem
I: cleaning the build env
I: removing directory /var/cache/pbuilder/build//14027 and its subdirectories
Traceback (most recent call last):
  File "/usr/bin/pbuilderjenkins", line 92, in <module>
    builder.execute_build(hooks=args.hook_list)
  File "/usr/lib/python2.7/dist-packages/pbuilderjenkins/pbuilderjenkins.py", line 321, in execute_build
    self._build(hooks_dir)
  File "/usr/lib/python2.7/dist-packages/pbuilderjenkins/pbuilderjenkins.py", line 287, in _build
    subprocess.check_call(command)
  File "/usr/lib/python2.7/subprocess.py", line 511, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['pdebuild', '--', '--inputfile', 'ReportDir', '--bindmounts', 'results', '--basetgz', '/var/cache/pbuilder/precise-amd64.tgz', '--buildresult', 'output', '--hookdir', '/tmp/tmpU6pGsn']' returned non-zero exit status 1
pbuilder returned ERROR.