Fallback to NullClient if initializing server fails

[vinistock]

Closes #265

Currently, if we fail to initialize the server, we show errors every time hover occurs. The reason is because initialize raises, meaning the memoized client is never set and thus causing us to try to initialized it again on every hover.

Since the client is bound to be used everywhere in this addon, I think the easiest way of handling it is falling back to a NullClient that simply no-ops most operations. That way, we don't have to spread a bunch of conditionals everywhere in the code.

Manual tests

1. On this branch, change the Open3.popen3 command from bin/rails to something that doesn't exist. Like non-existing-command-test.
2. Reload VS Code
3. Try to hover on a constant
4. Verify that an error gets printed to the output tab
5. Hover again on a constant
6. Verify nothing gets printed