Error handling

[BasileiosKal]

Implicitly handle errors form the utility sub-routines in the core operations + address point B from #246

[BasileiosKal]

Updated the PR with the following

• Introduced ABORT (like in h2c). An operation will ABORT if some of the Precoditions are not met. Any operation calling a function that ABORTed will also ABORT.
• If expand_message ABORTs (which it may), hash_to_scalar will also ABORT, so no need for checking explicitly its result (or require expand_len to be defined in a way that will not cause expand_message to fail, which may not always be possible).
• return INVALID is used for errors in the Deserialization/Procedure (like getting an invalid signature or deserializing a malformed proof). A subroutine's INVALID result is explicitly handled by the calling operation.
• Updated hash_to_scalar and create_generators to NOT check for 0, duplicates etc.

IMO this gives the more readable result.

[BasileiosKal]

Breaking Changes

1. In hash_to_scalar removed the counter
2. In create_generators step 3, changed I2OSP(i, 4) to I2OSP(i, 8)

("1." is because I removed the checks for 0 and "2." is to align the max number of messages with the max number of generators)

[tplooker]

Ready for merge, subject to resolving the conflicts.