Refine FormHttpMessageConverter exception handling

FormHttpMessageConverter could throw a more specific
HttpMessageNotReadableException instead of an IllegalArgumentException
when the http form data is invalid.

See gh-34594
Signed-off-by: Russell Bolles <rbolles@netflix.com>

spring-web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

@@ -353,12 +353,22 @@ public class FormHttpMessageConverter implements HttpMessageConverter<MultiValue
 		for (String pair : pairs) {
 			int idx = pair.indexOf('=');
 			if (idx == -1) {
-				result.add(URLDecoder.decode(pair, charset), null);
+				try {
+					result.add(URLDecoder.decode(pair, charset), null);
+				}
+				catch (IllegalArgumentException ex) {
+					throw new HttpMessageNotReadableException("Could not decode HTTP form payload", ex, inputMessage);
+				}
 			}
 			else {
-				String name = URLDecoder.decode(pair.substring(0, idx), charset);
-				String value = URLDecoder.decode(pair.substring(idx + 1), charset);
-				result.add(name, value);
+				try {
+					String name = URLDecoder.decode(pair.substring(0, idx), charset);
+					String value = URLDecoder.decode(pair.substring(idx + 1), charset);
+					result.add(name, value);
+				}
+				catch (IllegalArgumentException ex) {
+					throw new HttpMessageNotReadableException("Could not decode HTTP form payload", ex, inputMessage);
+				}
 			}
 		}
 		return result;