Sub-issue description

We show a variety of error messages when adding domain managers (in both enterprise and legacy mode). Many of these aren't descriptive, or are confusing, which we noticed during testing for enterprise mode. We need to update several of them.

12 total error messages need to be updated. This spreadsheet lists the location, current content (column F), and updated content (column I) of all 12.

Acceptance criteria

• 12 total messages are updated accordingly

Additional context

• More context about the actions, triggers and locations of the error messages can be found in this Mural board (let @witha-k know if you need access). The messages that have a red asterisk (*) are the messages that need to be updated in this ticket. The messages on the board include an ID (ex. "dm-add-alert001") in brackets. That ID is also listed in the spreadsheet (column A).

• After this dev ticket is completed, product should consider whether we should continue design work to improve the error messages in the other 11 flows.

Related issues

• 🔄 Related to the bug described in Display alerts that emails successfully and unsuccessfully sent to domain managers when a domain manager is removed #3944
• 🔄 Related to Email failures: Review current behavior for error handling & messaging #3476