pkgset: Handle exceptions in pkgset threads

There are two thread pools for making package sets. If Pungi is being
terminated by external event and the exception is handled in the first
thread, the second one never gets to the `stop` method and the process
keeps hanging.

This patch should make sure that `stop()` is called on both pools.

JIRA: RHELCMP-2459
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/pkgset/common.py

@@ -271,7 +271,12 @@ class MaterializedPackageSet(object):
 
         small_pool.start()
         big_pool.start()
+        try:
             small_pool.stop()
+        except Exception:
+            big_pool.kill()
+            raise
+        finally:
             big_pool.stop()
 
         return small_pool.results + big_pool.results