Error page regarding unknown usage requirements

docs/err/unknown-usage.rst

@@ -0,0 +1,15 @@
+Error: Unknown Usage/Linking Requirements
+#########################################
+
+A library can declare that it *uses* or *links* to another library by using the
+``Uses`` and ``Links`` keys in ``library.dds``, respectively.
+
+These requirements are specified by using the ``Namespace/Name`` pair that
+identifies a library. These are defined by both the project's dependencies and
+the project itself. If a ``Uses`` or ``Links`` key does not correspond to a
+known library, ``dds`` will not be able to resolve the usage requirements, and
+will generate an error.
+
+To fix this issue, ensure that you have correctly spelled the library
+identifier and that the package that contains the respective library has been
+declared as a dependency of the library that is trying to use it.