Raise ImportError instead of exiting, if buildbot.test can't be imported.

This apparently causes issue with 'pydoc -k', and is in general not nice.
Trial still fails with a sensible error message.

This is a slave-side reflection of commit 126e02341c76bd5a027c2ca5a8467aa5708228f2
don't run sqlalchemy betas, either
Don't claim to obfuscate passwords with master-side source step.

We don't currently have any way of obfuscating arbitrary things sent from the
master, so don't claim that we can.
Fixed try builds not being scheduled when more than one is pending
Merge branch 'buildbot-0.8.7'