Omit backtraces by default

Backtrace output is now omited unless "-d" or "--debug" is specified. Example:

• With debug: 1560851081.299585:ERROR:While executing command "run" Traceback (most recent call last): File "/home/okinst/.local/bin/kpet", line 11, in <module> load_entry_point('kpet', 'console_scripts', 'kpet')() File "/home/okinst/projects/kpet/kpet/__init__.py", line 76, in main exec_command(args, commands) File "/home/okinst/projects/kpet/kpet/__init__.py", line 31, in exec_command command[0](*command[1:]) File "/home/okinst/projects/kpet/kpet/cmd_run.py", line 256, in main raise Exception("\"{}\" is not a database directory".format(args.db)) Exception: ".../kpet-db" is not a database directory
• Without debug: 1560851172.384047:ERROR:While executing command "run" Exception: ".../kpet-db" is not a database directory