Introduce/use JSONFormatToInvalidSignatureError

[kolyshkin]

Based on a patch from #2512.

Feel free to close if it doesn't make much sense (I'm on the verge).

[mtrmac]

I’m not sure this is worth it as such, either, but on balance, let’s do it:

• The unrelated typo fixes are certainly valuable :)
• Rule of thumb, “deduplicate when there are 3 copies”
• Ideally, we’d like some kind of “unmarshaling is an InvalidSignatureError” helper/mix-in, but the only way I know how to do that in Go would be

  type signatureUnmarshaler struct { underlying interface { strictUnmarshalJson(…)…}
  type UntrustedRekorSET { signatureUnmarshaler; … }

  … which costs an extra interface pointer for this.UntrustedRekorSET.signatureUnmarshaler.underlying == this, and doesn’t work at all for UnmarshalJSON because that doesn’t call any construct which would set underlying.