Fix error handling on Add New Member form when the user already belongs to the portfolio

[rachidatecs]

Current Behavior

The current error is:
"This user is already assigned to a portfolio. Based on current waffle flag settings, users cannot be assigned to multiple portfolios." (in the case of a user)

Or
"This user is already assigned to a portfolio invitation. Based on current waffle flag settings, users cannot be assigned to multiple portfolios." (in the case of a non-user)

Expected Behavior

The errors should be:
"< email > is already a member of another .gov organization." (in the case of a user)

Or
"< email > has already been invited to another .gov organization." (in the case of a non-user)

These errors should trigger regardless of the multiple_portfolio flag setting

Steps to Reproduce

1. Invite an existing portfolio user to the portfolio
2. Invite an already invited non-user email to the portfolio

Environment

No response

Additional Context

No response

Issue Links

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