Tag creation: Throws a 500 server exception

[amarsan]

I was trying to create a tag for a concept set. I gave the tag a name, tag group, and icon. I clicked save and got a 500 response from the server.

[p-hoffmann]

The 500 comes from WebAPI rather than Atlas 3, by two separate routes.

1. GlobalExceptionHandler.handleDataIntegrityViolation assumes the cause chain is two levels deep, carries a message, and contains a PostgreSQL Detail: section. When that does not hold it throws a NullPointerException out of the handler itself, so the intended 409 surfaces as a 500. Tags reach this because tags_name_idx is a unique index on lower(name) over the whole tag table, and tag groups are rows in that same table (TagGroupService persists them through the same Tag entity and repository), so a name colliding with any existing tag or group lands there.
2. TagService.create throws IllegalArgumentException when a selected group has allow_custom = false. Nothing handles that type, so it falls to the generic handler and becomes a 500.

Fix opened as OHDSI/WebAPI#2539. Once it lands Atlas 3 shows the real reason without changes, as the HTTP client already surfaces the server's message.