Improve the error message on mis-use of context url

[Kreyren]

The current message on mis-used context url alike https://gitpod.io/#https://githfasfasub.com/gitpod-io/gitpod/issues/1505 is:

proposing:

---

We are having trouble parsing your context url

FATAL: The context url 'https://githfasfasub.com/gitpod-io/gitpod/issues/1505' is not recognized by gitpod.

This usually happens when:

1. Typographical error
2. Mishandling of context url for example using of 'https://gitpod.io/#https://something.nothing/gitpod-io/gitpod' instead of using supported repository hosters like GitHub 'https://gitpod.io/#https://github.com/gitpod-io/gitpod'

---

Originally posted by @Kreyren in #1505 (comment)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.