mypy: Switch to follow_imports = error.

The mypy daemon only supports error and skip. Choosing error
immediately surfaces when a file excluded from the build is
being imported, so that something can be done about it.

mypy.ini

@@ -18,10 +18,11 @@ local_partial_types = True
 # someone fixes a type error we don't want to add a burden for them.
 #warn_unused_ignores = True
 
-# Options to make the checking *less* strict, which we
-# might ideally eliminate.
-follow_imports = silent
-
+# Error on importing modules that are present but not part of the
+# build.  If the module can't reasonably be made to not error, errors
+# can be suppressed with ignore_errors.
+# (The mypy daemon only supports error and skip for follow_imports)
+follow_imports = error
 
 #
 #