Improve error messages when front-matter is copy-pasted incorrectly

Closes #25

packages/literate-elm/src/program.ts

@@ -375,6 +375,14 @@ const convertErrorsToMessages = (
 };
 
 const getErrorMessageText = (error): string => {
+  if (error.title === "UNKNOWN IMPORT") {
+    // Unknown imports form a special case. It is not reasonable to suggest looking into elm.json and source-directories
+    // as this is causing confusion in the literate-elm environment.
+    const failedImport = _.get(error, ["message", 1, "string"]);
+    if (failedImport) {
+      return `Could not ${failedImport}. Please make sure you have specified all dependencies on third-party Elm modules.`;
+    }
+  }
   if (_.isArray(error.message)) {
     const text = error.message
       .map((chunk) => {

packages/litvis/src/document/frontmatter/lintElm.ts

@@ -31,5 +31,16 @@ export default (dataWithPosition, document: LitvisDocument): void => {
         );
       });
     }
+    if (dataWithPosition.dependencies && (!elm || !elm.dependencies)) {
+      const elmPosition = getPosition(dataWithPosition.elm);
+      const dependenciesPosition = getPosition(dataWithPosition.dependencies);
+      if (dependenciesPosition.start.line > elmPosition.end.line) {
+        document.message(
+          `It seems that you are missing indentation before ‘dependencies’ and its sub-nodes. Without leading spaces, the value is not considered as a child node of ‘elm’.`,
+          dependenciesPosition,
+          "litvis:frontmatter:elm:dependencies",
+        );
+      }
+    }
   }
 };