requestbackport: Error out nicely when a tracking project doens't exist.

LP: #1852901
Signed-off-by: Mattia Rizzolo <mattia@debian.org>

debian/changelog

@@ -3,6 +3,8 @@ ubuntu-dev-tools (0.175) UNRELEASED; urgency=medium
   [ Mattia Rizzolo ]
   * Trust the installed debian-keyring when checking validity of dsc
     signatures.
+  * requestbackport:
+    + Error out nicely when a tracking project doens't exist.  LP: #1852901
 
   [ Stefano Rivera ]
   * merge-changelog: rewrite the changelog handling to use python3-debian.

requestbackport

@@ -90,7 +90,14 @@ def check_existing(package, destinations):
     query = re.findall(r'[a-z]+', package)
     bugs = []
     for release in destinations:
-        project = Launchpad.projects[release + '-backports']
+        project_name = '{}-backports'.format(release)
+        try:
+            project = Launchpad.projects[project_name]
+        except KeyError:
+            Logger.error("The backports tracking project '%s' doesn't seem to "
+                         "exist.  Please check the situation with the "
+                         "backports team.", project_name)
+            sys.exit(1)
         bugs += project.searchTasks(omit_duplicates=True,
                                     search_text=query,
                                     status=["Incomplete", "New", "Confirmed",