GHCi debugger: new flag -fbreak-on-exception

GHCi debugger: new flag -fbreak-on-exception

When -fbreak-on-exception is set, an exception will cause GHCi to
suspend the current computation and return to the prompt, where the
history of the current evaluation can be inspected (if we are in
:trace).  This isn't on by default, because the behaviour could be
confusing: for example, ^C will cause a breakpoint.  It can be very
useful for finding the cause of a "head []" or a "fromJust Nothing",
though.