replace definition of error with errorWithStackTrace

[Carter Schonwald]

http://hackage.haskell.org/package/base-4.7.0.1/docs/GHC-Stack.html#v:errorWithStackTrace

has been in GHC for for >=1 major version,
and for normal builds, acts just like error, BUT, when an error is thrown
in a profiled build, it appends a stack trace with some basic source
location data to the end of the error message!

I think this change would benefit many!

My current understanding is that the DWARF based approach to stack traces
wont make it into  GHC 7.10 , and while this variant would only provide
extra info in profiling builds (and strictly less than the dwarf work), its
something that can definitely be done in a single small patch that could
easily be swapped out for that improved approach once it lands.


discussion period: 2 weeks unless theres a clear unanimous agreement to
make the change ASAP, OR if the stack trace error stuff is landing in 7.10
and I'm misinformed!

cheers
-Carter
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