quick fix for missing task error reporting

the reason string given to the exception contained
characters that are forbidden in an http status
header (it had a newline).

in theory, we could put the multi-line description
in the body of the error response, but that involves
a few larger api changes and seems to also line up
best for exception semantics where the name of the
exception class is a decent summary and then the
message is always treated as multi-line detailed
content. However, our exception classing is an even
thornier issue so this suffices for now