Add descriptive hints to DASP connection errors

Authentication failures now show "check username and password", busy
devices say "device is busy", etc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dasp.py

@@ -101,9 +101,18 @@ def _trace(direction, msg, datagram=None):
     print(f"[DASP] {ts} {direction} {name}{seq}{ack} payload={plen}{hexdump}")
 
 
+_ERROR_HINTS = {
+    ERR_NOT_AUTHENTICATED: " - check username and password",
+    ERR_BUSY: " - device is busy",
+    ERR_INCOMPATIBLE_VERSION: " - incompatible protocol version",
+    ERR_DIGEST_NOT_SUPPORTED: " - digest algorithm not supported",
+}
+
+
 class DaspException(Exception):
     def __init__(self, msg, error_code=-1):
-        super().__init__(msg)
+        hint = _ERROR_HINTS.get(error_code, "")
+        super().__init__(msg + hint)
         self.error_code = error_code