Consider storing a more direct source_location in error_codes

[grisumbras]

Currently error_code stores the location of the most inner code that sets the error (the place where the error is first discovered). But there's an argument, that it should instead store the topmost location where the error is discovered.

Peter Dimov:

the point here is that in general, the location should point to the call that failed, not inside of it
if an OS API call fails, the location points to where it was issued, not to the OS source
here the call that fails is try_as_array
not something called inside of it