Improve error message when start and rollack fails

[kvch]

When pgroll fails to execute start phase of the migration, it runs rollback immediately. If rollback fails as well, the errors are joined. This made the error message hard to understand.

Previously, pgroll emitted this message:

Error: unable to execute start operation: pq: relation "qsdf" does not exist
pq: current transaction is aborted, commands ignored until end of transaction block

From now on, the message is clearer.

Error: unable to execute start operation of 'my_migration_name': pq: relation "qsdf" does not exist
unable to roll back failed operation: pq: current transaction is aborted, commands ignored until end of transaction block

If start fails but rollback succeeds, we emit a clearer error message:

Error: rolled back 'my_migration_name' because start command has failed: pq: relation "qsdf" does not exist