pickerWrapper had logic very similar to status.FromContextError() for
transforming Context errors to status errors. This patch removes the
duplication by delegating to the status library. Besides removing the
code duplication, the status library is arguably more robust because it
doesn't rely on ctx.Error() to only ever return two types of errors.

I believe this patch and the previous one stand on their own, but, FWIW,
they're also motivating by me wanting to experiment in the CockroachDB
codebase with using a custom implementation of context.Context whose
Err() method can return better errors than the stdlib context.Context.
These errors would still wrap context.Canceled.  Such an implementation
would technically break the documentation of context.Context, which
seems to exhaustively list the sentinel error that context.Context can
return. Still, as grpc#4977 showed, most
code should support wrapped context errors. This patch moves from "most
code" to "all code" in gRPC. I haven't checked which of the callsites
I've touched use contexts that might be inherited from a gRPC client, as
opposed to contexts derived inside gRPC from context.Background (which
contexts would not be affected by whatever I do outside of gRPC), but
unifying all the context error handling code seems like a good idea to
me universally.