(core) Show proper message on empty Excel import, rather than a code error

Summary:
- Previously showed "UnboundLocalError". Now will show:
    Import failed: Failed to parse Excel file.
    Error: No tables found (1 empty tables skipped)
- Also fix logging for import code

Test Plan: Added a test case

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3396

app/server/lib/DocPluginManager.ts

@@ -141,7 +141,7 @@ export class DocPluginManager {
         '.csv' : 'CSV',
       };
       const fileType = extToType[path.extname(fileName)] || path.extname(fileName);
-      throw new Error(`Failed to parse ${fileType} file. Error: ${messages.join("; ")}`);
+      throw new Error(`Failed to parse ${fileType} file.\nError: ${messages.join("; ")}`);
     }
     throw new Error(`File format is not supported.`);
   }