fix(python/trezorctl): improve docs and behavior around unexpected messages

closes #2254

python/src/trezorlib/cli/device.py

@@ -53,7 +53,10 @@ def cli() -> None:
 @cli.command()
 @with_client
 def self_test(client: "TrezorClient") -> str:
-    """Perform a self-test."""
+    """Perform a factory self-test.
+
+    Only available on PRODTEST firmware.
+    """
     return debuglink.self_test(client)
 
 
@@ -126,6 +129,7 @@ def load(
         if not label:
             label = "SLIP-0014"
 
+    try:
         return debuglink.load_device(
             client,
             mnemonic=list(mnemonic),
@@ -137,6 +141,13 @@ def load(
             needs_backup=needs_backup,
             no_backup=no_backup,
         )
+    except exceptions.TrezorFailure as e:
+        if e.code == messages.FailureType.UnexpectedMessage:
+            raise click.ClickException(
+                "Unrecognized message. Make sure your Trezor is using debug firmware."
+            )
+        else:
+            raise
 
 
 @cli.command()