Differentiate request_csr endpoint error codes for semantically invalid email vs other errors

[jacobshandling]

When POSTing to /fleet/mdm/apple/request_csr, the endpoint should return 4.xx (422?) for semantically invalid emails instead of the current 5.xx.

background:

the UI needs to see two options for error responses:

• 4.xx for either:
  • syntactically invalid email (also will be validated client side)
  • semantically invalid email, i.e., an email that is not allowed for this purpose
• 5.xx for anything else

The endpoint currently returns 422 for syntactically invalid emails, but 5.xx for semantically invalid emails, as well as for other server errors.

[lukeheath]

@eashaw Do you have the capacity to take this on this week?

[lukeheath]

@eashaw My mistake; this is on the Fleet API side, not fleetdm.com. Please disregard.

[fleet-release]

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