[merge] More helpful errors when missing a dependency

[Martin Pool]

On  7 Jul 2006, John Arbash Meinel <john at arbash-meinel.com> wrote:
> 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

+1, please merge it, and please add a NEWS entry including the bug
numbers.

(Is it really worthwhile?  I guess it's strictly redundant when the
source is version controlled, but I do fairly often look at them in
installed software, so I think it's worth keeping it up.)

-- 
Martin