fix: Improve gentx validation & error messages

[anilcse]

Description

Closes: #9304

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[michaelfig]

Looks like it provides the expected specific panics instead of generic panics. Thanks!

[amaury1093]

Nice! Let's also add a changelog message

[atheeshp]

lgtm

[robert-zaremba]

let's return errors instead of panicing. The function signature already supports that

[anilcse]

@robert-zaremba updated the PR, can you please take a final review

[robert-zaremba]

utACK