Stylistic change.

I doubt [catching_error] is performance critical in any way. But it looked silly to allocate a block to [(inner_trace,e)] when [e] was known in advance (and was already named [e]).
Fix printing of Ltac's backtrace.

I used an exception wrapper to report Tactic failures. It had the consequence of making process_vernac_interp_error to look for the backtrace at the wrong place.