[LSP] lazily compute callstacks on errors
[LSP] lazily compute callstacks on errors

Summary: when an LSP command errors, we sometimes pass along a stack trace. if not, we use the current call stack. so, only compute the current callstack when we don't already have a stack trace.

Reviewed By: gabelevi

Differential Revision: D17839794

fbshipit-source-id: 80d20ce3bc9d948163de03ce499d7c399937f802

facebook/flow,gabelevi/flow,facebook/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,facebook/flow,samwgoldman/flow,facebook/flow,gabelevi/flow,facebook/flow,mroch/flow,facebook/flow,gabelevi/flow,mroch/flow,mroch/flow,gabelevi/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,mroch/flow,mroch/flow,gabelevi/flow,gabelevi/flow