fixing return types in crypto module documentation

[jtfirek]

Description

I am a student at the PBA and I came across this issue when completing the first assignment as I was reading the rust docs. It looks a few functions in this file are documented as if they have an Option return type when they are actually returning a Result

• What does this PR do? Fixes the documentation for 3 functions by changing returns a None to returns an Error
• Why are these changes needed? I think the documentation is incorrect
• How were these changes implemented and what do they affect? They will change the rust documentation

[cla-bot-2021[bot]]

User @jtfirek, please sign the CLA here.

[ggwpez]

Polkadot companion: At PBA

Why does this need a companion? That line makes the CI fail btw.

[bkchr]

bot merge

[paritytech-processbot[bot]]

Waiting for commit status.