Add error handling to previewQueryHelp

Author: mgsium

extensions/ql-vscode/src/extension.ts

@@ -498,12 +498,21 @@ async function activateWithInstalledDistribution(
   ): Promise<void> {
     const path = selectedQuery ? selectedQuery.path : window.activeTextEditor!.document.uri.fsPath;
     if(path) {
+      // Get path of temporary 'qh-markdown' directory in extension context
       const tempDirPath = `${getContextStoragePath(ctx)}/qh-markdown/`;
+      // Get library name and query file name in the format 'libname/queryname.md
       const mdFilePath = path.split('/').slice(-2).join('/').replace('.qhelp', '.md');
-      const absolutePath = `${tempDirPath}${mdFilePath}`;
-      const uri = Uri.file(absolutePath);
-      await cliServer.generateQueryHelp(path , 'markdown', absolutePath);
-      await commands.executeCommand('markdown.showPreviewToSide', uri);
+      const absolutePathToMd = `${tempDirPath}${mdFilePath}`;
+      const uri = Uri.file(absolutePathToMd);
+      try {
+        await cliServer.generateQueryHelp(path , 'markdown', absolutePathToMd);
+        await commands.executeCommand('markdown.showPreviewToSide', uri);
+      } catch (err) {
+        const errorMessage =  err.message.includes('Generating qlhelp in markdown') ? (
+          `Could not generate markdown from ${path}: Bad formatting in .qhelp file.`
+        ) : `Could not open a preview of the generated file (${absolutePathToMd}).`;
+        void helpers.showAndLogErrorMessage(errorMessage);
+      }
     }
   }