Don't keep input for error messages

Don't keep input for error messages

When generating error messages, the raw JSValue is rarely
useful. However, keeping it for error messages---even if
only in the unused branch of an if statement---prevents this
value from going out of scope.

Note: with the smaller number of arguments in the readJSONWithDesc
function, newer versions of ghc try too fancy optimisations and thus
run out of memory; hence we have to reduce the ghc optimisation level
for some files.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Oleg Ponomarev <oponomarev@google.com>