fix(calendar): fall back to stdout when readpst's stderr is empty

readpst writes some failure messages to stdout rather than stderr, so a
failing run with empty stderr surfaced as a blank "parse failed: " error
with no diagnostic content.

src-tauri/src/calendar/mod.rs

@@ -79,9 +79,15 @@ fn run_readpst(pst_path: &str, out_dir: &Path) -> Result<(), CalError> {
             _ => CalError::Open(e.to_string()),
         })?;
     if !output.status.success() {
-        return Err(CalError::Parse(
-            String::from_utf8_lossy(&output.stderr).trim().to_string(),
-        ));
+        // readpst writes some errors to stdout rather than stderr; show
+        // whichever stream actually has text, stderr first.
+        let stderr = String::from_utf8_lossy(&output.stderr).trim().to_string();
+        let message = if stderr.is_empty() {
+            String::from_utf8_lossy(&output.stdout).trim().to_string()
+        } else {
+            stderr
+        };
+        return Err(CalError::Parse(message));
     }
     Ok(())
 }