Improve error message on unexpected message types

**Citrus Version**
v2.7.6

**Scenario**
Citrus receives a message of type plaintext where the expected message type for validation is XML. The resulting error message is misleading and not pointing to the actual unexpected message type.

The current error message is:

> Could not find proper message validator for message type 'xml', please define a capable message validator for this message type

Improve the error message so it is clear that the validation could not perform because of message type mismatch.

Start by reproducing the plaintext-versus-XML validation scenario described for Citrus v2.7.6, then trace where the current validator error is assembled. Done means the error clearly identifies the message-type mismatch rather than suggesting that an XML validator is missing.

Written by the indexing model from the issue text.