dcrjson: update error types.

[dnldd]

This updates the dcrjson error types to leverage go 1.13 errors.Is/As functionality as well as confirm to the error infrastructure best practices outlined in #2181.

[davecgh]

The removal of the jsonerr.go and associated errors isn't correct. These are error codes with very specific meaning to JSON-RPC that need to be retained.

Note that the old ErrorCode cases are for the package API and those should be switched over (as you've done here). However, the "Standard JSON-RPC 2.0 errors", "General application defined JSON errors", etc, need to remain because those are codes that are required by the JSON-RPC specification.

[davecgh]

To follow up, I believe jsonerr.go will want a new

type RPCError struct {
	Code int
	Message string
}

Then all of those errors would become:

ErrInvalidRequest = RPCError{
...
}

...

You can see from the removed comment that there was once a separation between the two for this purpose:

// Error identifies a general error.  This differs from an RPCError in that this
// error typically is used by the consumers of the package as opposed to
// RPCErrors which are intended to be returned to the client across the wire via
// a JSON-RPC Response.  The caller can use type assertions to determine the
// specific error and access the ErrorCode field.

I guess it was incorrectly changed at some point to shoehorn it into using the generic Error type when it shouldn't have been.

[davecgh]

Just needs the copyright year updated on the modified files and then this is good to go.