[merge] More helpful errors when missing a dependency

[John Arbash Meinel]

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In response to this bug:
https://launchpad.net/products/bzr/+bug/47821
or this one
https://launchpad.net/products/bzr/+bug/52204

And similar ones that keep cropping up, I created the attached bundle.
Since we now support having multiple Transports implement handling a
given protocol, I know just keep track of a 'last error', and if I am
unable to bind to a specific URL, I raise UnsupportedProtocol with that
error.

The old code used to try to use 'sftp://' as a local path (which was
very bad). After my encoding branch landed, it changed to reporting:

bzr: ERROR: Invalid url supplied to transport:
'sftp://localhost/~/,,xxx': Unsupported protocol: sftp

Which was better, but didn't tell you anything about why sftp was
supported. After the attached patch, the error changes to:

bzr: ERROR: Unsupported protocol for url "sftp://localhost/~/,,test":
Unable to import paramiko (required for sftp support): No module named
paramiko

Which may be a little bit too verbose, but I believe it should help
reduce the number of bug reports that we get. It not only tells them
what was wrong with the path they supplied, but I think it gives them a
hint as to how to fix it. (And if the module was just mis-configured,
then it would probably tell them that as well).

John
=:->

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