Add a proper message to IncludedFeaNotFound

Also, minor f-string refactor.

Lib/fontTools/feaLib/error.py

@@ -9,10 +9,18 @@ class FeatureLibError(Exception):
         message = Exception.__str__(self)
         if self.location:
             path, line, column = self.location
-            return "%s:%d:%d: %s" % (path, line, column, message)
+            return f"{path}:{line}:{column}: {message}"
         else:
             return message
 
 
 class IncludedFeaNotFound(FeatureLibError):
-    pass
+    def __str__(self):
+        assert self.location is not None
+
+        message = (
+            "The following feature file should be included but cannot be found: "
+            f"{Exception.__str__(self)}"
+        )
+        path, line, column = self.location
+        return f"{path}:{line}:{column}: {message}"

Tests/feaLib/lexer_test.py

@@ -178,7 +178,9 @@ class IncludingLexerTest(unittest.TestCase):
     def test_include_missing_file(self):
         lexer = IncludingLexer(self.getpath("include/includemissingfile.fea"))
         self.assertRaisesRegex(IncludedFeaNotFound,
-                               "includemissingfile.fea:1:8: missingfile.fea",
+                               "includemissingfile.fea:1:8: The following feature file "
+                               "should be included but cannot be found: "
+                               "missingfile.fea",
                                lambda: list(lexer))
 
     def test_featurefilepath_None(self):