LetsEncryptClientError
Less generic exception for subprocess.check_call.

https://docs.python.org/2/library/subprocess.html#exceptions
Less generic exception catching in send()
Merge remote-tracking branch 'github/master' into bugs/44

Conflicts:
	letsencrypt/client/client.py
Merge remote-tracking branch 'github/master' into bugs/44

Conflicts:
	letsencrypt/client/apache_configurator.py
Raise errors.LetsEncryptClient error in send().
Merge pull request #71 from kuba/bugs/44

Towards better exceptions...