9fb5edb39e ecdsa: Improve error types (Tobin C. Harding)
Pull request description:
There are a couple of issues around the ECDSA signature decoding / parsing code. We have duplicate code in `from_str` and `from_slice` and both use the same error type even though it is impossible to get a hex error in `from_slice`.
Create two errors:
- A `DecodeError` returned by `from_slice`
- A `FromStrError` that has a decode variant and a hex variant
Call through to `from_slice` after parsing hex into a byte vector.
Removes an instance of `unreachable!`.
Fix: #1193
ACKs for top commit:
Kixunil:
ACK 9fb5edb39e
apoelstra:
ACK 9fb5edb39e successfully ran local tests
Tree-SHA512: 3b3ae31887d603f1739d261b491b99f7847987f94dbbfefb9aa84d4250736eba2d007d28746bbb064946d3055e4cca01510677bf2cdbb11bbf83d7388dbd2620
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