Add text to excuses in case autopkgtest is delayed

In case autopkgtest triggering is delayed because the required builds aren't
ready yet or the package is not installable, currently there is only the
message that autopktest delays the migration, but no hint why. This commit adds
these hints.

britney2/policies/autopkgtest.py

@@ -235,10 +235,12 @@ class AutopkgtestPolicy(BasePolicy):
         binaries_info = self.suite_info[suite].sources[source_name]
         if not binaries_info.binaries:
             self.logger.info('%s hasn''t been built anywhere, skipping autopkgtest policy', excuse.name)
+            excuse.addhtml("nothing built yet, autopkgtest delayed")
             verdict = PolicyVerdict.REJECTED_TEMPORARILY
 
         if 'all' in excuse.missing_builds:
             self.logger.info('%s hasn''t been built for arch:all, skipping autopkgtest policy', source_name)
+            excuse.addhtml("arch:all not built yet, autopkgtest delayed")
             verdict = PolicyVerdict.REJECTED_TEMPORARILY
 
         if verdict == PolicyVerdict.PASS:
@@ -254,9 +256,11 @@ class AutopkgtestPolicy(BasePolicy):
                 if arch in excuse.missing_builds:
                     verdict = PolicyVerdict.REJECTED_TEMPORARILY
                     self.logger.info('%s hasn''t been built on arch %s, delay autopkgtest there', source_name, arch)
+                    excuse.addhtml("arch:%s not built yet, autopkgtest delayed there" % arch)
                 elif arch in excuse.unsatisfiable_on_archs:
                     verdict = PolicyVerdict.REJECTED_TEMPORARILY
                     self.logger.info('%s is uninstallable on arch %s, delay autopkgtest there', source_name, arch)
+                    excuse.addhtml("uninstallable on arch %s, autopkgtest delayed there" % arch)
                 else:
                     # request tests (unless they were already requested earlier or have a result)
                     tests = self.tests_for_source(source_name, source_data_srcdist.version, arch)