Produce useful error message for imageplan failures
https://illumos.org/issues/10169 is a case where a package which had a
trailing whitespace in its manifest 'license' line caused tracebacks
when installing unrelated packages.
This change causes error messages like this to be produced instead:
pkg install: The requested operation cannot be completed due to invalid package metadata. Details follow:
pkg://firstname.lastname@example.org,5.11-2018.0.0.0:20181224T183734Z has invalid manifest line:
'license 3e7ce9223a3d638f969ee023155c70c84b28b396 license="GPLv3, FDLv1.3 "'
'GPLv3, FDLv1.3 ' != 'GPLv3, FDLv1.3'