CI: check for ERROR string in output

Let the CI test fail on any error string, such as:

lib/apk/exec/postmarketos-base-28-r0.trigger: ERROR: deviceinfo file is missing!!
ERROR: postmarketos-base-28-r0.trigger: script exited with error 1

Let's actually fix that there is an error message first in pmaports, backport it to branches tested by this, and then merge this.

@craftyguy: would you like to take a look at this?

Fix #12