spec out error codes and messages for ACH API

[matin]

Document meaning of different status codes and errors. For now, the status code alone should be sufficient to indicate the error and revisit specific error messages later.

Possible errors

• missing field
• value does not follow proper format
• malformed request. Unknown fields
• trying to set or update a read only field
• incorrect routing number
• not enough funds in escrow/reserve to issue a credit
• unauthorized request. Invalid api key
• API server error