refactor(auth): better error message for method handler panic

[technicallyty]

Description

we panic when we attempt to register a msg that does not implement sdk.Msg. However, this is not immediately apparent in the panic error string.

This PR updates the panic to be more specific as to why it panicked.

Closes: n/a

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[alexanderbez]

LGTM

[likhita-809]

lgtm