tools/mpremote: Print nicer errors for unsupported 'cp -r' arguments.

Also document support for 'cp :a :b'.

Signed-off-by: Damien George <damien@micropython.org>

docs/reference/mpremote.rst

@@ -262,6 +262,8 @@ Examples
 
   mpremote cp main.py :
 
+  mpremote cp :a.py :b.py
+
   mpremote cp -r dir/ :
 
   mpremote cp a.py b.py : + repl

tools/mpremote/mpremote/main.py

@@ -333,10 +333,15 @@ def do_filesystem(pyb, args):
     if fs_args[0] == "cp" and fs_args[1] == "-r":
         fs_args.pop(0)
         fs_args.pop(0)
-        assert fs_args[-1] == ":"
+        if fs_args[-1] != ":":
+            print(f"{_PROG}: 'cp -r' destination must be ':'")
+            sys.exit(1)
         fs_args.pop()
         src_files = []
         for path in fs_args:
+            if path.startswith(":"):
+                print(f"{_PROG}: 'cp -r' source files must be local")
+                sys.exit(1)
             _list_recursive(src_files, path)
         known_dirs = {""}
         pyb.exec_("import uos")