[plt-scheme] exception handling

In debugging my SchemeQL problem, I realized belatedly that if the exception
had not been being trapped by SchemeQL, I would have been able to click the
bug icon, or the little piece of paper icon (what do you call that?) and see
which call was generating the exception.  This would have saved me quite
some time (and spared everyone a few emails!)

I tried changing the 'else' case in 'sister-interaction' to simply (raise
exn), instead of:

  (raise (make-schemeql-unknown-error (wrap-srpersist-exception henv hdbc
hstmt)))

...and sure enough, when the error occurs I get a bug icon, along with a
message which reads "Unspecified error in col-attribute", and I can get a
backtrace.

I'll know better next time.  One thing I don't quite understand, though, is
why the original 'raise' call above does *not* generate a bug icon.  Is it
the wrong kind of exception, or perhaps it's being trapped again somewhere
higher up?  If there were some way to toggle this behavior, it would be
useful.

Anton