Improve possible customization for invalid signup field error messages

[imath]

The hooks to customize invalid signup field error messages are relying on an anonymous function which only include the error message HTML output. As the filter to customize this message bp_members_signup_error_message is not dynamic, to make sure to customize the message for a specific signup field, we need to add a parameter to transport the field name.

[imath]

In 12875:

Improve possible customization of invalid signup field error message

Adds a new parameter ($fieldname) to the anonymous function firing the bp_members_signup_error_message and includes it into the filter params.

It's now easier to customize the error message according to the concerned invalid signup field name.

Fixes #8449