Bug 1618011 part 1. Implement support for error context in ThrowErrorMessage. r=peterv

This does not change behavior at the moment, because the only callers of
ThrowErrorMessage that pass an error number that has a context pass an empty
string for the first (context) arg. Both of those callers are changed to pass
nullptr for the context in this patch.

We want to support nullptr to mean "empty context", because that way at
callsites we can avoid having extra empty strings.

We could avoid putting this machinery in place if we hardcoded the trailing
": " at all the callsites, but that would reduce future flexibility in where the
context is placed in the message string (e.g. if we wanted to move it to the
end instead of the beginning) and increase the amount of string data we have to
cart around in the binary quite noticeably: we have a _lot_ of places in
bindings where we call ThrowErrorMessage.

Differential Revision: https://phabricator.services.mozilla.com/D64882