Added helpful error message when patch_file is used without patch

[runfalk]

Currently diesel print-schema will print not found if patch is not installed and the patch_file argument is used. This bit me as I thought it could not find my patch file (which wasn't the case).

Hopefully this patch helps others in my position.

[sgrif]

The change seems fine, but I don't think you should need to reformat all the code underneath it for such a small change.

[runfalk]

There we go. I've made the change less invasive.

[JohnTitor]

Thanks! CI is red due to unrelated failing, I re-approve this.

[weiznich]

Could you rebase this PR on top of #2131 to fix the CI?

[runfalk]

It seems like there is still some issues with the CI. I've looked at both the failing Travis and Azure pipelines logs and it seems to be timeouts unrelated to my changes. I've retriggered the build.

[runfalk]

The re-run sems to have fixed Azure, but Travis is still not playing nice. If I look at the failed logs they don't event get to setting up Rust.

[weiznich]

Seems like travis has some issues in the last time regarding to downloading stuff. Hopefully the will fix that soon.
(Anyway no need to wait for this.)