Improve error message for routing

pkgs/code_assets/lib/src/code_assets/validation.dart

@@ -295,10 +295,10 @@ ValidationErrors _validateFile(
 }) {
   final errors = <String>[];
   if (mustBeAbsolute && !uri.isAbsolute) {
-    errors.add('$name (${uri.toFilePath()}) must be an absolute path.');
+    errors.add('$name at ${uri.toFilePath()} must be an absolute path.');
   }
   if (mustExist && !File.fromUri(uri).existsSync()) {
-    errors.add('$name (${uri.toFilePath()}) does not exist as a file.');
+    errors.add('$name at ${uri.toFilePath()} does not exist as a file.');
   }
   return errors;
 }

pkgs/data_assets/lib/src/data_assets/validation.dart

@@ -31,7 +31,7 @@ List<String> _validateHookInput(List<EncodedAsset> assets) {
     final dataAsset = DataAsset.fromEncoded(asset);
     errors.addAll(
       _validateFile(
-        'LinkInput.assets.data asset "${dataAsset.id}" file',
+        'LinkInput.assets.data asset "${dataAsset.id}"',
         dataAsset.file,
       ),
     );
@@ -98,7 +98,7 @@ void _validateDataAsset(
     errors.add('More than one data asset with same "${dataAsset.name}" name.');
   }
   final file = dataAsset.file;
-  errors.addAll(_validateFile('Data asset ${dataAsset.name} file', file));
+  errors.addAll(_validateFile('Data asset "${dataAsset.name}"', file));
 }
 
 ValidationErrors _validateDataAssetSyntax(EncodedAsset encodedAsset) {
@@ -129,10 +129,12 @@ ValidationErrors _validateFile(
 }) {
   final errors = <String>[];
   if (mustBeAbsolute && !uri.isAbsolute) {
-    errors.add('$name (${uri.toFilePath()}) must be an absolute path.');
+    errors.add(
+      '$name points to "${uri.toFilePath()}", which must be an absolute path.',
+    );
   }
   if (mustExist && !File.fromUri(uri).existsSync()) {
-    errors.add('$name (${uri.toFilePath()}) does not exist as a file.');
+    errors.add('$name points to "${uri.toFilePath()}", which does not exist.');
   }
   return errors;
 }

pkgs/hooks/lib/src/validation.dart

@@ -151,9 +151,26 @@ class ProtocolBase {
     final errors = <String>[];
     if (!input.config.linkingEnabled) {
       if (output.assets.encodedAssetsForLinking.isNotEmpty) {
-        const error =
-            'BuildOutput.assets_for_linking is not empty while '
-            'BuildInput.config.linkingEnabled is false';
+        final error =
+            '''
+`BuildOutput.assets_for_linking` is not empty while `BuildInput.config.linkingEnabled` is `false`.
+
+The assets ${output.assets.encodedAssetsForLinking} where sent to linking, but should either be bundled with the app or linked only when linking is enabled.
+
+This might be caused by writing something like
+
+```
+routing: ToLinkHook(input.packageName),
+```
+
+Try writing this instead:
+
+```
+routing: input.config.linkingEnabled
+              ? ToLinkHook(input.packageName)
+              : ToAppBundle(),
+```
+''';
         errors.add(error);
       }
     }