Add an heptapod explanation to the GitLab remote message that is displayed when push fails.

When I tried to push to create MR, it failed.

Here is the message I got when it failed:

openpyxl/openpyxl!382 (comment 141022)

$ hg push
pushing to ssh://hg@foss.heptapod.net/openpyxl/openpyxl
remote: agent key RSA SHA256:fujwINTBl3D9qq01JmNAAqyYVxk54BdIDg4o+VCqqo4 returned incorrect signature type
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: user "heptapod-write" is not authorised to publish changesets: xxx

abort: push failed on remote

I would like you to add an explanation to the remote message so that the user who pushed knows what to do. For example, I think the following message would be easier to understand:

remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: user "heptapod-write" is not authorised to publish changesets: xxx
remote: For more information on creating MR, see https://heptapod.net/pages/quick-start-guide.html#quick-start-guide.  <--- ADDED

abort: push failed on remote

I think this will help users who are unfamiliar with hg and users who are unfamiliar with the hg topic.