give a better error message for missing envvar

[chadwhitacre]

@jeromegn got burned over on #1157 when he rebased his local branch on master, because we had added a dependency on a new key in local.env and of course he didn't have it:

pid-60504 thread-140735195500928 (MainThread) Oh no! Aspen crashed!
pid-60504 thread-140735195500928 (MainThread) Traceback (most recent call last):
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/env/lib/python2.7/site-packages/aspen/server.py", line 125, in main
pid-60504 thread-140735195500928 (MainThread)     _main(argv)
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/env/lib/python2.7/site-packages/aspen/server.py", line 51, in _main
pid-60504 thread-140735195500928 (MainThread)     website = Website(argv)
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/env/lib/python2.7/site-packages/aspen/website.py", line 30, in __init__
pid-60504 thread-140735195500928 (MainThread)     self.configure(argv)
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/env/lib/python2.7/site-packages/aspen/configuration/__init__.py", line 382, in configure
pid-60504 thread-140735195500928 (MainThread)     execfile(filepath, {'website': self})
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/configure-aspen.py", line 90, in <module>
pid-60504 thread-140735195500928 (MainThread)     UPDATE_HOMEPAGE_EVERY = int(os.environ['UPDATE_HOMEPAGE_EVERY'])
pid-60504 thread-140735195500928 (MainThread)   File "/Users/jgn/projects/www.gittip.com/env/lib/python2.7/UserDict.py", line 23, in __getitem__
pid-60504 thread-140735195500928 (MainThread)     raise KeyError(key)
pid-60504 thread-140735195500928 (MainThread) KeyError: 'UPDATE_HOMEPAGE_EVERY'
^Cpid-60504 thread-140735195500928 (MainThread) Received INT, exiting.

Our design is to require all keys in every *.env file. We don't have defaults, because then the risk is greater of leaking development settings into production or vice versa, depending on which defaults we pick. What we can do, however, is give a nicer error message when a key is missing.