Improve import error handling and messages

[mikkohei13]

• Should show clear error messages about all fields, for example:
  • if import file is empty, shows "must have required property 'gatherings'". Should say something clearer, e.g. "file is empty" or "location data missing", depending when exactly this error is shown.
  • If Havainnoijat field contains a MA-code that doesn't exist, shows error "leg". Should show "Unknown observer identifier MA....".
• Should not get stuck when (??) See https://luomus-ict.slack.com/archives/C01QYRXB62H/p1767084968656699