better handling of json.error

[pinheadmz]

This issue is similar to bcoin-org/brq#7 in that I'm getting unexpected behavior when using bcurl for APIs besides bcoin.

See here:
https://github.com/bcoin-org/bcurl/blob/master/lib/client.js#L211-L217

This gets broken by Kraken's API, which is already frustrating for a few other reasons, but one of them is this -- the API always returns an error property in the JSON:

https://api.kraken.com/0/public/Ticker?pair=XBTUSD
-->

{"error":[],"result":{"XXBTZUSD":{"a":["3536.10000","2","2.000"],"b":["3535.30000","1","1.000"],"c":["3531.80000","1.58333841"],"v":["8259.75520827","9590.27330937"],"p":["3510.71489","3507.83774"],"t":[12317,14569],"l":["3433.90000","3433.90000"],"h":["3579.80000","3579.80000"],"o":"3502.00000"}}}

Even though the error array is empty, bcurl detects that the property is there and throws without returning the rest of the object.

[BluSyn]

Solved in #10

[braydonf]

So bcurl may not be that useful in other such cases. The error handling is fairly specific to how the error is formatted and how bcoin and bweb returns it. This library should probably have a different name, merge with brq (#18) to correspond as the client for bweb, or an alternative.