Error codes for spec-tests

[iurii-ssv]

Resolves #514 to implement error-code-based checks in tests (to replace error-message-based ones),

the general approach I took is:

• go over all the tests that assert on the expected error message
• "turn that error-message into an error-code" and assert on the error-code instead (the implementation returns an error that has both code and message, in spec-tests only the error-code is asserted on to allow for updating error-messages without tripping spec-tests - this is mostly useful for upgrading consensus code in https://github.com/ssvlabs/ssv without having to update the mirroring error-messages in this repo)
• there are some errors that are not checked for by any of the tests, and hence that means those errors are left unchanged (I did not create a corresponding error-code for those) ... I think it's OK for now (if somebody adds a test that needs such an error to have an error-code it would be easy to add following the approach I'm using in this PR)
• all error-codes are listed in types/error.go file
• besides the changes to spec-tests (and their corresponding generated .json files), the only other notable change this PR introduces is to split the invalid partial sig slot error (in validatePartialSigMsgForSlot func) into 2 distinct errors invalid partial sig slot and future partial sig msg to be able to differentiate between messages we want to retry vs messages we want to drop (as is implemented in runners: robust message replay ssv#2445)

[GalRogozinski]

Good job!

[y0sher]

LGTM, but this also means enforcing adding an error code each time an error is added.