Be more informative if launching a Jetpack branch fails due to the branch not being part of a Pull Request

[oskosk]

We only allow branches on Pull Requests to be used on the Jetpack Beta Tester Plugin (which is the basis for Jurassic Ninja's branch feature when asking for a site with Jetpack Beta Tester plugin jetpack-beta)

[sirreal]

It would be nice to error if one of the requested branches can't be found, whether Calypso or Jetpack, rather than sliently booting with master.

[simison]

Alternative "fix": show the current Jetpack+Calypso branch at the wp-admin bar.

[oskosk]

It would be nice to error if one of the requested branches can't be found, whether Calypso or Jetpack, rather than sliently booting with master.

This, I think, happened only when the gutenpack feature was first launched. A few bugs came up as it was a dirty hack. JN won't silently launch with master of any of the repos if the requested branches are not there.

The intention of this issue was mostly about tracking the need for informing whether a branch is not a PR because currently JN just shows a message stating that the branch probably hasn't been built yet instead of just asserting "It's not a PR".