Upgrade: improve handling of unserialize errors

Merging PR https://github.com/mantisbt/mantisbt/pull/1359

In MantisBT 1.3, the method used to store config, token and filter data
changed from serialize to JSON. Upgrade steps 193, 194 and 195 were
added to convert legacy data to the new format.

This frequently caused the upgrade process to fail when unserialize()
could not interpret the data, but the MantisBT installer just aborted
without providing any useful information to the admin.

This improves the error reporting, clearly identifying the offending
record so that the admin can take appropriate action to fix the problem.

Fixes 0024416, 0021376 (and maybe others too)