fix type of locs in Error_message.ESignatureVerification
fix type of locs in Error_message.ESignatureVerification

Summary: `Error_message.aloc_of_msg` extracts an `ALoc` from `ESignatureVerification` errors. I want to generalize `aloc_of_msg` to work on either `ALoc.t` or `Loc.t`, so when we have a `Loc.t Error_message.t'`, `ESignatureVerification` needs to contain `Loc.t Signature_error.t`.

Reviewed By: dsainati1

Differential Revision: D18061979

fbshipit-source-id: f0406fd7d48569d552d1a749606cdefa3d248f30

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