[hack] a bit more logging around exceptions
[hack] a bit more logging around exceptions

Summary:
I observed an assertion failure at this line when trying to use emacs lsp-mode.

By switching from "assert" to "failwith", I expect the exception message will have more information, so richer information will go into the telemetry logs.

Reviewed By: arxanas

Differential Revision: D5342037

fbshipit-source-id: e66cc72192bd339fb73490bb96881cffe675eb6a

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