trigger: make trigger_run() exception safe

To convert engine and space infrastructure to C, we need to make txn
usable from C code. The only reason why we can't convert txn.cc to C
right now is triggers: they are set from alter.cc and may throw
exceptions. To handle this, let's make trigger_run() catch all
exceptions and return an error code instead. For C++ code, introduce
trigger_run_xc() which calls trigger_run() under the hood and throws
the exception suppressed by it.

Needed for #2776