SPFx: project upgrade should return a state beside error and log

[StfBauer]

Currently, in the callback of the project upgrade, only log and error get returned. This allows the following scenarios.

Case 1: Cannot downgrade: current project is newer than the generator - ERR filled but not LOG
Case 2: All good: Current project matches generator - ERR filled but not LOG
Case 3: Upgrade required: Current project is older and needs to be upgraded - ERR is empty but LOG is filled

For case 1 and case 2, it would be great if this can return an error code to make a distinctive decision based on the error code.
For example:
ERROR CODE: 1 - Good to go no upgrade required
ERROR CODE: -1 - Downgrade not supported.

[waldekmastykarz]

This is a good point, especially when you think of integrating CLI in other solutions and need more information than a faulty error code.

[waldekmastykarz]

@StfBauer could you please check if #631 has the information you'd need?