Continue resolving dependencies rather than bailing on first error

Continue resolving dependencies rather than bailing on first error

This allows error messages emitted by the frontend to be a bit more
descriptive and not have the annoying "well why didn't you tell me that
the first time" problem. If a package had multiple missing deps, we
would bail on the first one before rather than finish processing all
missing dependencies, and only print one error message. Instead,
continue through this entire set of missing deps and append all eventual
errors.

The added pactest tests this case, as the to be installed package has
two missing dependencies. However, pactest does not actually test or see
the difference in output from before and after, so it passes in both
cases, but it is clearly visible in the logs.

Signed-off-by: Dan McGee <dan@archlinux.org>