mkrepo.sh: bail out if packages are not installable

... instead of producing a broken local mirror. Also provide a hint
what exactly went wrong (which package had unmet dependencies, etc).

Closes-bug: #1368137

Change-Id: Ifa7e86d706bea025b2deb3f9ace1be2909110e72
Merge "mkrepo.sh: bail out if packages are not installable"