Inconsistent error names

[tcharding]

The PsbtParseError and ParseAmountError are inconsistent names. We have many other parsing errors and they are typically called ParseError then users can use one level of path to qualify if they wish e.g., psbt::ParseError would be more consistent.

[apoelstra]

Yeah, we could change these ones. But I don't think in general we can do this -- e.g. I would expect the PSBT module to have multiple kinds of parse errors. In rust-minscript we definitely have this situation.

I also don't really think this matters ... if you are explicitly naming errors in your code then you need to be reading through docs anyway because they're named inconsistently throughout the entire ecosystem. But if you want to make this change, I'm cool with it.

[tcharding]

No worries, thanks.