SDK: Don't reconnect to bblfsh or analyzer

[smacker]

Part of #29

[carlosms]

This is great if you are developing an analyzer that doesn't need UAST, but if you do need it, and fail to connect lookout to bblfsh properly, the current code will ignore req.WantUAST silently, right?

A single warning at startup will be easy to miss if the requests themselves return success, but with unexpected contents. I think a request with wantUAST should fail, with an error message that informs that lookout could not connect to bblfsh.

[carlosms]

Another option I can think of is to make --bblfshd optional, and if it is set make lookout try to connect once, print the error and exit.

[bzz]

👍 on @carlosms suggestions, it's important to get SDK UX right, so users have trust it in.
Let's address it.

[smacker]

I would prefer to implement in separate PR to unblock sequential PRs and don't make this PR too huge. Here is an issue: #45

[bzz]

SGTM, with handling improvements in separate PR