Show output of pg_dump / pg_restore commands in case of failures

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The rake task gitlab:db:decomposition:migrate is using pg_dump and pg_resture. If something goes wrong with those command (for exmaple, the ci database is not found), they fail silently and we show a 'migration finished' message

If one of those command fails, we should:

• print the output of the command
• exit the rake task with exit code 1
• Print something like 'Migration failed'