Improve error message with invalid repository configurations

[Emmanuel Blot]

There are several issues related to invalid repository configuration errors:

1. when repository_type is defined, but repository_dir is left undefined, a confusing error message is thrown:

   Unsupported version control system "svn". 
   Check that the Python bindings for "svn" are correctly installed.
   

   exactly as if SVN Python bindings were in trouble.
   This message may even be more confusing as as repository_type defaults to svn, a configuration that does not contain any repository_* directives ends up showing up this error message.
2. HTTP error type shows up as None when this error is raised.

[Christian Boos]

The vc backends should be able to bring more actual details about the failure.

[Chris Mulligan <chris.mulligan@…>]

This is a dupe of #6796 which has a lot more detail.