Always return something on request

[giltho]

Changed the signature of request to return either a success (store * resp) or a Response.Error
The server should always answer requests, even in case of error, it should return an error response.

[rgrinberg]

Looks good

[rgrinberg]

One issue this PR introduced is the mass spamming of errors like this in Emacs:

LSP :: Invalid_argument("get_every_pattern: [{\"filename\":\"ocaml.ml\",\"start\":{\"line\":379,\"col\":32},\"end\":{\"line\":379,\"col\":40},\"ghost\":false,\"attrs\":[],\"kind\":\"value_binding\",\"children\":[{\"filename\":\"ocaml.ml\",\"start\":{\"line\":379,\"col\":36},\"end\":{\"line\":379,\"col\":40},\"ghost\":false,\"attrs\":[],\"kind\":\"pattern (ocaml.ml[379,10448+36]..ocaml.ml[379,10448+40])\\n  Tpat_var \\\"poly/17813\\\"\\n\",\"children\":[]},{\"filename\":\"ocaml.ml\",\"start\":{\"line\":379,\"col\":40},\"end\":{\"line\":379,\"col\":40},\"ghost\":true,\"attrs\":[{\"start\":{\"line\":379,\"col\":40},\"end\":{\"line\":379,\"col\":40},\"name\":\"merlin.hole\"}],\"kind\":\"expression\",\"children\":[]}]}]") [8 times]

and like this:

LSP :: Failure("No node at given range") [7 times]

[giltho]

urgh
I'm trying to understand if these errors are legitimate.
This is the server answering every request, as it should, and not silencing any issue. I guess Emacs is showing the errors ?
Now, are these bugs that I introduced or are those just errors that were there before but were invisible ?

[giltho]

The "get_every_pattern" error seems to be coming from a wrong call to get_every_pattern in merlin/src/analysis/destruct. This happens after calls to code_actions being fired
I think the source is the same for the "No node given at range" thing which is due to the case_analysis call in code actions

Let's track this in an issue