Improve implementation code for requestVerifiablePresentation

[lmuntaner]

Motivation

Clean up the code to make it more maintainable and readable.

Changes

• Change implementation of requestVerifiablePresentation by using only one handler on the message event and a conditional logic inside.

Tests

• Only one test needed changing and it makes sense because the expected error was inconsistent with the other error.

[lmuntaner]

@frederikrothenberger please review when you have time.

It might be better to review the file on its own instead of the changes.

[frederikrothenberger]

LGMT, thanks. Left only minor comments.