improve open wallet error dialog message

src/main/java/com/sparrowwallet/sparrow/AppController.java

@@ -809,7 +809,7 @@ public class AppController implements Initializable {
                     } else {
                         if(!attemptImportWallet(file, password)) {
                             log.error("Error Opening Wallet", exception);
-                            showErrorDialog("Error Opening Wallet", exception.getMessage());
+                            showErrorDialog("Error Opening Wallet", exception.getMessage() == null ? "Unsupported file format" : exception.getMessage());
                         }
                         password.clear();
                     }
@@ -822,7 +822,7 @@ public class AppController implements Initializable {
         } catch(Exception e) {
             if(!attemptImportWallet(file, null)) {
                 log.error("Error opening wallet", e);
-                showErrorDialog("Error Opening Wallet", e.getMessage());
+                showErrorDialog("Error Opening Wallet", e.getMessage() == null ? "Unsupported file format" : e.getMessage());
             }
         }
     }

src/main/java/com/sparrowwallet/sparrow/io/Storage.java

@@ -222,12 +222,16 @@ public class Storage {
                 if(read != BINARY_HEADER_LENGTH) {
                     throw new StorageException("Not a Sparrow wallet - invalid header");
                 }
-                byte[] decodedHeader = Base64.getDecoder().decode(header);
-                byte[] magic = Arrays.copyOfRange(decodedHeader, 0, HEADER_MAGIC_1.length());
-                if(!HEADER_MAGIC_1.equals(new String(magic, StandardCharsets.UTF_8))) {
-                    throw new StorageException("Not a Sparrow wallet - invalid magic");
+                try {
+                    byte[] decodedHeader = Base64.getDecoder().decode(header);
+                    byte[] magic = Arrays.copyOfRange(decodedHeader, 0, HEADER_MAGIC_1.length());
+                    if(!HEADER_MAGIC_1.equals(new String(magic, StandardCharsets.UTF_8))) {
+                        throw new StorageException("Not a Sparrow wallet - invalid magic");
+                    }
+                    salt = Arrays.copyOfRange(decodedHeader, HEADER_MAGIC_1.length(), decodedHeader.length);
+                } catch(IllegalArgumentException e) {
+                    throw new StorageException("Not a Sparrow wallet - invalid header");
                 }
-                salt = Arrays.copyOfRange(decodedHeader, HEADER_MAGIC_1.length(), decodedHeader.length);
             } else {
                 SecureRandom secureRandom = new SecureRandom();
                 secureRandom.nextBytes(salt);