Fix validation logic

[atonks2]

These changes were made as a result of reviewing fedWireMessage side by side with the FedWire Format Reference Guide. They aim to accomplish two things:

1. Correct discrepancies between fedWireMessage.go and the spec referenced above
2. Document the requirements for each tag above its validation function

I think two followup tasks would be beneficial:

1. fedWireMessage handles validation of message tags with relation to the other tags, but relies on each tag's Validate() method to verify data integrity within that tag. We should conduct a review of each tag's Validate method to ensure it's in line with the spec.
2. The existing unit tests cover a lot of good cases, but we should make sure we have 100% coverage where compliance with the spec is concerned.
   • Related to this, nearly every unit test I encountered has its assertion(s) wrapped in conditional statements. These unit tests expect an error to be returned, then they assert that the actual error matches the expected error. The issue with this is if the system under test doesn't return an error, the test still passes even though it should fail.

[adamdecaf]

This is some awesome work, thanks so much!