Raise exception with error output if CalledProcessError when tagging.

[michaeleveringham]

Currently if tagging fails (ex: version were changed manually and an old tag was attempted to be re-used), subprocess.CalledProcessError is raised without helpful information.

Proposing changing this to raise a custom exception, TaggingFailureException, with the subprocess.CalledProcessError.returncode and .output to help diagnosis the issue.

This will also prevent a successful exit code from occurring if tagging fails, as noted in #224.

Ran make test successfully, except for Python 3.5 which is unavailable on the image:

____________________________________ ERROR collecting tests/test_cli.py ____________________________________
ImportError while importing test module '/code/tests/test_cli.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
.tox/py35/lib/python3.5/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_cli.py:15: in <module>
    from testfixtures import LogCapture
.tox/py35/lib/python3.5/site-packages/testfixtures/__init__.py:13: in <module>
    from testfixtures.comparison import (
.tox/py35/lib/python3.5/site-packages/testfixtures/comparison.py:14: in <module>
    from testfixtures.mock import parent_name, mock_call, unittest_mock_call
.tox/py35/lib/python3.5/site-packages/testfixtures/mock.py:47: in <module>
    sys.version_info, backport_version
E   ImportError: Please upgrade Python (you have sys.version_info(major=3, minor=5, micro=7, releaselevel='final', serial=0)) or Mock Backport (You have None)
========================================= short test summary info ==========================================
ERROR tests/test_cli.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
============================================= 1 error in 0.30s =============================================
ERROR: InvocationError for command /code/.tox/py35/bin/pytest -r a tests (exited with code 2)
...
_________________________________________________ summary __________________________________________________
ERROR:   py35: commands failed
  py36: commands succeeded
  py37: commands succeeded
  py38: commands succeeded
  pypy3: commands succeeded