Quit in postrelease when we cannot find a version.

[mauritsvanrees]

Fixes issue #262.

There were several checks like this already. I made them the same: do not compare with None, but check for a False value. That would catch a found but empty version as well.

[mauritsvanrees]

Travis passes, but there is an allowed error for PyPy. It passed on master. It seems to be caused by my change:

File "/home/travis/build/zestsoftware/zest.releaser/zest/releaser/tests/functional-with-hooks.txt", line 121, in functional-with-hooks.txt
Failed example:
    postrelease.main()
Exception raised:
    Traceback (most recent call last):
      File "/opt/python/pypy2.7-5.8.0/lib-python/2.7/doctest.py", line 1315, in __run
        compileflags, 1) in test.globs
      File "<doctest functional-with-hooks.txt[19]>", line 1, in <module>
        postrelease.main()
      File "/home/travis/build/zestsoftware/zest.releaser/zest/releaser/postrelease.py", line 106, in main
        postreleaser.run()
      File "/home/travis/build/zestsoftware/zest.releaser/zest/releaser/baserelease.py", line 351, in run
        self.prepare()
      File "/home/travis/build/zestsoftware/zest.releaser/zest/releaser/postrelease.py", line 49, in prepare
        sys.exit(1)
      File "/home/travis/build/zestsoftware/zest.releaser/zest/releaser/tests/functional.py", line 36, in _exit
        raise RuntimeError(msg)
    RuntimeError: SYSTEM EXIT (code=1)

[mauritsvanrees]

Okay, previously PyPy passed on master, but that seems just luck. There are earlier runs that fail in the same way. And I have restarted the PyPy job and I get a similar error but at a different spot. So the PyPy job is indeed unstable. Would be nice to get that fixed, but that is not for this PR. I will merge.