Give a better error message for packages on bitbucket.org
Give a better error message for packages on bitbucket.org

bitbucket.org doesn't support the go-import meta tag protocol, which
lead to an obscure error message

'go get' has special-case support for bitbucket.org.  bitbucket.org
hosts both hg and git repos, and 'go get' uses the bitbucket API to
discover which command to use.  Vendetta could do the same, in order to
support git repos on bitbucket.org (obviously it can't support projects
with hg repos).  But no-one has asked for that yet.

dpw/vendetta