(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/client/components/PluginScreen.ts

@@ -111,6 +111,7 @@ const cssModalBody = styled('div', `
   padding: 16px 0;
   overflow-y: auto;
   max-width: 470px;
+  white-space: pre-line;
 `);
 
 const cssModalHeader = styled('div', `