rpcserver: Cleanup getvoteinfo RPC.

[davecgh]

This makes the getvoteinfo RPC handling more consistent with the rest of the handlers and corrects the return error type for unrecognized vote versions.

Fixes #1953.

[chappjc]

Sorry I missed giving a review, but how about a rpc server version bump?

[davecgh]

Do you really think it's necessary? The only thing that changed is it now returns the proper error code (invalid parameter) instead of an internal error. The actual behavior of the RPC is identical in that it either returns the same information when provided with a known version, or it returns an error when it's not.

I didn't really think that warranted a version bump, but if it causes issues, we can certainly do that.

[chappjc]

Just because we're using a regexp here https://github.com/chappjc/dcrdata/blob/8ce7befd0af3d4f47a5d6c5c2675a9ba996f9595/gov/agendas/deployments.go#L124
and changing to a type assertion and code check is desirable. But we'd want to recognize if dcrd supports it.

[davecgh]

Bumped in #1970