[decorators] Add raise_on_error decorator

Click turns functions into "commands" which has the following consequences:
- Any return statement means that it has succeeded whatever the value returned
  therefore the exit code will be 0
- To "fail" properly an exception must be raised which will return an exit
  code of 1
- Other exceptions will be treated as usage error and have an exit code
  of 2

The raise_on_error decorator will take the returned value of the wrapped
function and raise a generic error with the exit code set to what the
function has returned.

beat/cmdline/decorators.py

+# vim: set fileencoding=utf-8 :
+
+###############################################################################
+#                                                                             #
+# Copyright (c) 2016 Idiap Research Institute, http://www.idiap.ch/           #
+# Contact: beat.support@idiap.ch                                              #
+#                                                                             #
+# This file is part of the beat.cmdline module of the BEAT platform.          #
+#                                                                             #
+# Commercial License Usage                                                    #
+# Licensees holding valid commercial BEAT licenses may use this file in       #
+# accordance with the terms contained in a written agreement between you      #
+# and Idiap. For further information contact tto@idiap.ch                     #
+#                                                                             #
+# Alternatively, this file may be used under the terms of the GNU Affero      #
+# Public License version 3 as published by the Free Software and appearing    #
+# in the file LICENSE.AGPL included in the packaging of this file.            #
+# The BEAT platform is distributed in the hope that it will be useful, but    #
+# WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY  #
+# or FITNESS FOR A PARTICULAR PURPOSE.                                        #
+#                                                                             #
+# You should have received a copy of the GNU Affero Public License along      #
+# with the BEAT platform. If not, see http://www.gnu.org/licenses/.           #
+#                                                                             #
+###############################################################################
+
+from functools import wraps
+
+import click
+
+
+def raise_on_error(view_func):
+    """Raise a click exception if returned value is not zero.
+
+    Click exits successfully if anything is returned, in order to exit properly
+    when something went wrong an exception must be raised.
+    """
+
+    def _decorator(*args, **kwargs):
+        value = view_func(*args, **kwargs)
+        if value not in [None, 0]:
+            exception = click.ClickException("Error occured")
+            exception.exit_code = value
+            raise exception
+        return value
+    return wraps(view_func)(_decorator)