Move UnsignedPackagesError to a separate file

This file can contain all Pungi specific exceptions.

It should also fix an issue encountered on Python 2.7:

    AttributeError: 'module' object has no attribute 'pkgsets'

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/errors.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+#
+# This program 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.  See the
+# GNU Library General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, see <https://gnu.org/licenses/>.
+
+
+class UnsignedPackagesError(RuntimeError):
+    """Raised when package set fails to find a properly signed copy of an
+    RPM."""
+
+    pass

pungi/phases/pkgset/pkgsets.py

@@ -33,10 +33,7 @@ from kobo.threads import WorkerThread, ThreadPool
 import pungi.wrappers.kojiwrapper
 from pungi.util import pkg_is_srpm, copy_all
 from pungi.arch import get_valid_arches, is_excluded
-
-
-class UnsignedPackagesError(RuntimeError):
-    pass
+from pungi.errors import UnsignedPackagesError
 
 
 class ExtendedRpmWrapper(kobo.pkgset.SimpleRpmWrapper):

pungi/scripts/pungi_koji.py

@@ -21,6 +21,7 @@ from six.moves import shlex_quote
 
 from pungi.phases import PHASES_NAMES
 from pungi import get_full_version, util
+from pungi.errors import UnsignedPackagesError
 
 
 # force C locales
@@ -335,7 +336,7 @@ def main():
             latest_link_status=latest_link_status,
             latest_link_components=latest_link_components,
         )
-    except pungi.phases.pkgset.pkgsets.UnsignedPackagesError:
+    except UnsignedPackagesError:
         # There was an unsigned package somewhere. It is not safe to reuse any
         # package set from this compose (since we could leak the unsigned
         # package). Let's make sure all reuse files are deleted.