app: Show stacktrace on exception when "--debug" is True

Description

When debugging, having the last stacktrace hidden is quite painful. Showing the stacktrace gives much better information both for the reporter and the developer.

Sample output with the change:

with --debug:

Error loading user configuration:

Traceback (most recent call last):
  File "src/buildstream/_yaml.pyx", line 298, in buildstream._yaml.load_data
TypeError: 'NoneType' object is not callable

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "src/buildstream/_yaml.pyx", line 275, in buildstream._yaml.load
  File "src/buildstream/_yaml.pyx", line 311, in buildstream._yaml.load_data
buildstream._exceptions.LoadError: Severely malformed YAML:

'NoneType' object is not callable



The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/c/Users/SchubertBenjamin/Documents/projects/buildstream/buildstream/src/buildstream/_frontend/app.py", line 180, in initialized
    self.context.load(config)
  File "/usr/lib/python3.6/contextlib.py", line 52, in inner
    return func(*args, **kwds)
  File "/c/Users/SchubertBenjamin/Documents/projects/buildstream/buildstream/src/buildstream/_context.py", line 213, in load
    defaults = _yaml.load(_site.default_user_config)
  File "src/buildstream/_yaml.pyx", line 257, in buildstream._yaml.load
  File "src/buildstream/_yaml.pyx", line 288, in buildstream._yaml.load
buildstream._exceptions.LoadError: /c/Users/SchubertBenjamin/Documents/projects/buildstream/buildstream/src/buildstream/data/userconfig.yaml: Severely malformed YAML:

'NoneType' object is not callable

Without --debug:

Error loading user configuration: /c/Users/SchubertBenjamin/Documents/projects/buildstream/buildstream/src/buildstream/data/userconfig.yaml: Severely malformed YAML:

'NoneType' object is not callable