Fix for inconsistent-return-statements errors from new pylint (2.7)

References:
- https://github.com/PyCQA/pylint/blob/5e04ce74faa0142fe3ff99eec08479cfaeb9584c/ChangeLog#L104
- pylint-dev/pylint#3468

lektor/buildfailures.py

@@ -52,6 +52,7 @@ def lookup_failure(self, artifact_name):
         except IOError as e:
             if e.errno != errno.ENOENT:
                 raise
+            return None
 
     def clear_failure(self, artifact_name):
         """Clears a stored failure."""

lektor/databags.py

@@ -24,6 +24,7 @@ def load_databag(filename):
     except (OSError, IOError) as e:
         if e.errno != errno.ENOENT:
             raise
+        return None
 
 
 class Databags:

lektor/utils.py

@@ -316,7 +316,7 @@ def locate_executable(exe_file, cwd=None, include_bundle_path=True):
                     return path + ext
         return None
     except OSError:
-        pass
+        return None
 
 
 class JSONEncoder(json.JSONEncoder):
@@ -356,6 +356,7 @@ def safe_call(func, args=None, kwargs=None):
     except Exception:
         # XXX: logging
         traceback.print_exc()
+        return None
 
 
 class Worker(Thread):
@@ -600,6 +601,9 @@ def get_relative_path(source, target):
         else:
             # prepend the distance to the common ancestor
             return distance / relpath
+    # We should never get here.  (The last ancestor in source.parents will
+    # be '.' — target.relative_to('.') will always succeed.)
+    raise AssertionError("This should not happen")
 
 
 def get_structure_hash(params):