feat: ECDSA error types

[ivokub]

Description

This PR returns error constants when ECDSA signature is invalid due to r^3 + ax + b being quadratic non-residue. This is used in gnark for handling edge cases. This also allows to make an internal method private.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[yelhousni]

good to merge 👍