Some more fixes

Author: lrandersson

constructor/briefcase.py

@@ -4,6 +4,7 @@
 
 import logging
 import re
+import sys
 import shutil
 import sysconfig
 import tempfile
@@ -137,6 +138,9 @@ def write_pyproject_toml(tmp_dir, info):
 
 
 def create(info, verbose=False):
+    if sys.platform != 'win32':
+        raise Exception(f"Invalid platform '{sys.platform}'. Only Windows is supported.")
+
     tmp_dir = Path(tempfile.mkdtemp())
     write_pyproject_toml(tmp_dir, info)