Produce a nice error when bsconfig uses unsupported features

I've got a build-system that excercises some of the … more obscure features of `bsb`.

I didn't necessarily expect reason-language-server to immediately *handle* all of them fully, but I also didn't expect it to straight crash. “Unable to read file://...excmd/bsconfig.json”

(I suspect either `"generators"` or `"ppx-flags"` is screwing it up? ¯\\\_(ツ)\_/¯)

Configuration in question: https://github.com/ELLIOTTCABLE/excmd.js/blob/master/bsconfig.json, and my [`node_modules/.lsp/debug.log`](https://gist.github.com/ELLIOTTCABLE/08195986d7c8c2242cf47ac658014bf1).

Start with the linked excmd.js bsconfig.json and its node_modules/.lsp/debug.log to reproduce the crash around the unsupported "generators" or "ppx-flags" features. Trace how the language server reads bsconfig.json and identify where the generic "Unable to read" failure is produced. Done means unsupported configuration is reported as a useful error instead of crashing.

Written by the indexing model from the issue text.