FIX pg_bindir on unofficial platforms

What does this merge request do and why?

pg_bindir has been changed to use GDK::PostgresqlUpgrader#available_versions and fallback to using pg_config in case of exception. But that doesn't work : #available_versions uses abort instead of raise, which is not rescueable, so support/pg_bindir exits with a non-zero status without printing anything.

Sadly, this causes gdk install to fail on non official platforms, right now (although, it's not the only reason :) ).

How to set up and validate locally

Given it's platform dependent, you can't reproduce the bug if you're on a supported platform. But you can observe the buggy behavior testing those scripts:

This will not be rescued:

begin
  abort 'Help!'
rescue StandardError
  puts 'saved!'
end

This will properly be rescued:

begin
  raise 'Help!'
rescue StandardError
  puts 'saved!'
end

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.