Replace all the various RPC 'bad parameter' error codes with one

[vinniefalco]

Instead of having rpcBAD_ISSUER, rpcBAD_MARKET, etc... we should just have one error code "rpcINVALID_PARAMS" (which already exists) and augment the returned JSON with information about which parameter is invalid, and why it is invalid. For example, we might add:

error_field : "taker_gets.currency"
error_problem : "parse error"

to indicate that the taker_gets["currency"] field in the JSON encountered a parsing error. This is much more robust and useful than having a proliferation of error codes. The other problem is that the current solution does not scale. Must we add a new error code for EVERY new JSON field? I sure hope not.