avocado.core.plugins: Print traceback on failure

On plugin import failure we get the error message, which in case of
IOError is: "IOError: [Errno 2] No such file or directory". We have to
debug it or add some prints to discover, what happened. Let's add
optional traceback print in the same fashion as the "loader" does.
(the output is ignored unless the user enables "avocado.app.tracebacks")
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/plugins/builtin.py

@@ -15,11 +15,13 @@
 """Builtin plugins."""
 
 import os
+import sys
 import logging
 from importlib import import_module
 
 from .plugin import Plugin
 from ..settings import settings
+from ...utils import stacktrace
 
 
 log = logging.getLogger("avocado.app")
@@ -59,6 +61,8 @@ def load_builtins():
             if name not in skip_notify:
                 log.error('Error loading %s -> %s', name, reason)
             ErrorsLoading.append((name, reason))
+            stacktrace.log_exc_info(sys.exc_info(),
+                                    'avocado.app.tracebacks')
             continue
         for name in plugin_mod.__dict__:
             obj = getattr(plugin_mod, name)