Added handling of bad return values from launching pluggable transport c...

...lients #2064
We don't use it anywhere else no. That tends to be my criteria for whether or not to create constants. Extracting them is kind of nice for self-documenting, but there's also something to be said for just seeing all the code in one line. To me that's about a tie, and the tie goes to less typing unless it's used in multiple places.