fix(detectors): Surface API error messages for cron detector creation

[malwilley]

Closes ISWF-2304

When creating a cron monitor with a slug that's already in use, the API returns a 400 with a useful error message nested inside dataSources.slug. But the frontend was just showing a generic "Unable to create monitor" toast, which isn't helpful.

This PR adds proper error surfacing in two ways:

1. For all monitor forms, extract the error message from the API response and show it in a toast
2. Specifically for the cron error, uses mapFormErrors to map the dataSources.slug error to the name field

For both of those points, I think that there are better ways we can handle these errors by supporting them in the base form components or by adding a better abstraction to our specific form to deal with the data source mapping. But until we refactor to the use the new form components, I think this is sufficient for now.

[linear-code]

ISWF-2304 Cron monitor: "name in use" errors are not surfaced to the user

[saponifi3d]

lgtm - thanks for including some todos around this too.

for the future - should we also try to standardize the API response (at least within workflow engine)? i think that might also help simplify the findFirstMessage function.

[malwilley]

for the future - should we also try to standardize the API response (at least within workflow engine)? i think that might also help simplify the findFirstMessage function.

Yes it certainly could! Maybe we don't need to return a keyed object at all, it could just be an array of error messages