Avoid hiding error context in VCS pip install.

When an error happens while trying to install a pip package from a VCS, like
git, if there's an '=' in the name, the error handling logic tried to be
clever and give a better error message, but that logic doesn't apply to VCS
names, which can legitimately have a single equal sign. So, in that case
report the original error message.
Merge pull request #8251 from heewa/pip_vcs_error_msg

Avoid hiding error context in VCS pip install.