Add error message if cert contains invalid UTF8 data

src/main.rs

@@ -65,7 +65,13 @@ fn parse_certificate(cert_file: String, date_format: String) -> CertParseResult
     for tmp in x509.subject_name().entries() {
         let cname = match tmp.data().as_utf8() {
             Ok(a) => a,
-            Err(_) => continue,
+            Err(e) => {
+                eprintln!(
+                    "Skipping invalid UTF8 data in cert file {}: {}",
+                    cert_file, e
+                );
+                continue;
+            }
         };
         cnames.push(cname.to_string());
     }