Log traceback in case of exceptions during optimizations (#1156)

So far, with `minimize(..., OptimizeOptions(allow_failed_starts=True))`
only the exception message was stored/logged, which is not always sufficiently
informative (see #1147).

Now we log the full exception type / message / traceback.

Closes #1147

pypesto/optimize/optimizer.py

@@ -94,7 +94,12 @@ def wrapped_minimize(
             )
         except Exception as err:
             if optimize_options.allow_failed_starts:
-                logger.error(f'start {id} failed: {err}')
+                import sys
+                import traceback
+
+                trace = "\n".join(traceback.format_exception(*sys.exc_info()))
+
+                logger.error(f'start {id} failed:\n{trace}')
                 result = OptimizerResult(
                     x0=x0, exitflag=-1, message=str(err), id=id
                 )