Improve error message for failed jitpack.io dependencies

[holly-cummins]

I used a github reference to trigger a jitpack dependency download. It didn't work, because it turned out that jitpack couldn't build my repository. The root cause wasn't obvious from the error message, which made it look like the build hadn't even checked in the jitpack repository:

Run jbangdev/[email protected]
...
[jbang] Resolving dependencies...
[jbang]    com.github.holly-cummins:github-api:0ba8970874
[jbang]    info.picocli:picocli:4.2.0
Error:  [ERROR] Could not resolve dependencies: Could not find artifact com.github.holly-cummins:github-api:jar:0ba8970874 in central (https://repo1.maven.org/maven2/)

It would be great if this would detect that I was trying to use a github repo and have a bit more guidance, something like "This is a GitHub link, and so Jitpack was also checked. To diagnose why the download didn't succeed visit https://jitpack.io/#holly-cummins/github-api to check the logs for the Jitpack build."

Note that jitpack isn't mentioned at all in the github action. When I run locally I do get a bit more information, because jitpack is listed in the repositories it tries:

[jbang] [ERROR] Could not resolve dependencies from mavencentral=https://repo1.maven.org/maven2/, jitpack=https://jitpack.io/
Unable to collect/resolve dependency tree for a resolution due to: Could not find artifact com.github.holly-cummins.github-api:HEAD:jar:main in mavencentral (https://repo1.maven.org/maven2/), caused by: Could not find artifact com.github.holly-cummins.github-api:HEAD:jar:main in mavencentral (https://repo1.maven.org/maven2/)