Easier to debug connect-to-bitbucket failure

Unlike on Github, the repository name on Bitbucket is case sensitive. This
gotcha is a bit counter-intuitive and therefore can lead to some head-scratching
as to why the migration script is failing. Improving the exception message makes
it easier to figure out what is going wrong.
Merge pull request #10 from c-w/master

Make it easier to trouble-shoot a failure to connect to bitbucket