Refine logging in StompErrorHandler

Avoid a full stacktrace at ERROR level for a client message that could
not be sent to a MessageChannel.

See gh-26026

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

@@ -328,8 +328,13 @@ else if (StompCommand.UNSUBSCRIBE.equals(command)) {
 			}
 			catch (Throwable ex) {
 				if (logger.isErrorEnabled()) {
-					logger.error("Failed to send client message to application via MessageChannel" +
-							" in session " + session.getId() + ". Sending STOMP ERROR to client.", ex);
+					String errorText = "Failed to send message to MessageChannel in session " + session.getId();
+					if (logger.isDebugEnabled()) {
+						logger.debug(errorText, ex);
+					}
+					else {
+						logger.error(errorText + ":" + ex.getMessage());
+					}
 				}
 				handleError(session, ex, message);
 			}