Factor-out the generation of "foo not available in path" comment messages into...

Factor-out the generation of "foo not available in path" comment messages into the exception that raises them.

diffoscope/comparators/utils/file.py

@@ -496,14 +496,8 @@ class File(metaclass=abc.ABCMeta):
                 if difference is None:
                     return None
                 difference.add_comment(
-                    "'%s' not available in path. Falling back to binary comparison."
-                    % e.command
+                    e.get_comment("Falling back to binary comparison.")
                 )
-                package = e.get_package()
-                if package:
-                    difference.add_comment(
-                        "Install '%s' to get a better output." % package
-                    )
             except OutputParsingError as e:
                 difference = self.compare_bytes(other, source=source)
                 if difference is None:

diffoscope/exc.py

@@ -34,6 +34,18 @@ class RequiredToolNotFound(Exception):
     def get_package(self):
         return get_package_provider(self.command)
 
+    def get_comment(self, infix=''):
+        xs = [
+            "'{}' not available in path.".format(self.command),
+            infix,
+        ]
+
+        x = self.get_package()
+        if x:
+            xs.append("Install '{}' to get a better output.".format(x))
+
+        return " ".join(x for x in xs if x)
+
 
 class ContainerExtractionError(Exception):
     def __init__(self, pathname, wrapped_exc):