* in some cases more specific sg exception types were used in place
of the more generic one, e.g., sg_io_exception instead of sg_exception
when the context of the error was an IO error
* in some cases, the error message was made more specific
* minor style fix for exception rethrowing --- using throw; whenever
a re-throw is made; sometimes optimizing away the exception symbol name
in the catch handler at all
* more specific catch handlers added in some places -- e.g.,
an sg_io_exception caught ahead of sg_exception
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