[ticket/11394] Relax Migration Tools

Do not throw as many exceptions in the migration tools (when something
unexpected occurs but can be safely ignored).

PHPBB3-11394
Merge remote-tracking branch 'EXreaction/ticket/11394' into develop

# By Nathaniel Guse
# Via Nathaniel Guse
* EXreaction/ticket/11394:
  [ticket/11394] Relax Migration Tools