Unhelpful error messages when inviting an email address and you don't have an identity server configured

### Steps to reproduce

1. Use a session with no ID server configured
2. Click "Start chat"
3. Enter email address in search box (as the dialog suggests, cf #14699)
4. Press Go

Alternatively:

1. Use a session with no ID server configured
2. In an existing room, click "Invite"
3. Enter email address in search box (as the dialog suggests, cf #14699)
4. Press Go

### Outcome

#### What did you expect?

Some sort of prompt telling me I can't invite email addresses without an identity server

#### What happened instead?

or

Neither of these errors are at all helpful. "Invites by email can only be sent one at a time" is particularly confusing, given I was in fact only inviting one user.

### Operating system

_No response_

### Browser information

_No response_

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

No