Throw Error when rate limits are hit

[christroutner]

When bch-js receives a 429 error from bch-api, it sometimes returns the error message as an object, rather than throwing an error. The proper behavior is to throw an error.

The scope of this Issue is to review the error handlers of endpoints and properly handle 429 errors. This code snippet of validateTxid3() captures how it catches and properly handles 429 errors:

• Handle 429 rate limit errors