fix backtrace printed when cannot get lock. change lock error handling code so exceptions have useful info and exception handling in one place. add test case for when cannot get lock.

Catch HTTPException when reading from remote http repository. If the server dies very early, an httplib.IncompleteRead exception may be raised, because httplib can't read a single byte. Catching all HTTPException subclasses here will prevent ugly backtraces for similar things, too.

merge with crew.