Error page for package layout issues

docs/err/invalid-pkg-filesystem.rst

+Error: Invalid package filesystem
+#################################
+
+``dds`` prescribes a filesystem structure that must be followed for it to
+build and process packages and libraries. This structure isn't overly strict,
+and is thoroughly explained on the :doc:`/guide/packages` page.
+
+..  .
+  TODO: Create are more detailed reference page for package and library layout,
+  and include those links in a `seealso`.

src/dds/error/errors.cpp

         return "invalid-version-string.html";
     case errc::invalid_config_key:
         return "invalid-config-key.html";
+    case errc::invalid_lib_filesystem:
+        return "invalid-pkg-filesystem.html";
     case errc::none:
         break;
     }
 )";
     case errc::invalid_config_key:
         return R"(The `key' in a `key: value' pair was not recognized.)";
+    case errc::invalid_lib_filesystem:
+        return R"(
+`dds` prescribes a specific filesystem structure that must be obeyed by
+libraries and packages. Refer to the documentation for an explanation and
+reference on these prescriptions.
+)";
     case errc::none:
         break;
     }
     case errc::invalid_config_key:
         return "Found an invalid configuration key. <- (Seeing this text is a `dds` bug. Please "
                "report it.)";
+    case errc::invalid_lib_filesystem:
+        return "The filesystem structure of the package/library is invalid. <- (Seeing this text "
+               "is a `dds` bug. Please report it.)";
     case errc::none:
         break;
     }

src/dds/error/errors.hpp

     invalid_version_range_string,
     invalid_version_string,
     invalid_config_key,
+
+    invalid_lib_filesystem,
 };
 
 std::string      error_reference_of(errc) noexcept;

src/dds/library/library.cpp

 #include <dds/library/library.hpp>
 
 #include <dds/build/plan/compile_file.hpp>
+#include <dds/error/errors.hpp>
 #include <dds/source/dir.hpp>
 #include <dds/util/algo.hpp>
 
 
     if (include_dir.exists()) {
         if (!fs::is_directory(include_dir.path)) {
-            throw std::runtime_error("The `include` at the root of the project is not a directory");
+            throw_user_error<errc::invalid_lib_filesystem>(
+                "The `include` at the library root [in {}] is a non-directory file", path.string());
         }
         auto inc_sources = include_dir.collect_sources();
         // Drop any source files we found within `include/`
 
     if (src_dir.exists()) {
         if (!fs::is_directory(src_dir.path)) {
-            throw std::runtime_error("The `src` at the root of the project is not a directory");
+            throw_user_error<errc::invalid_lib_filesystem>(
+                "The `src` at the library root [in {}] is a non-directory file", path.string());
         }
         auto src_sources = src_dir.collect_sources();
         extend(sources, src_sources);