fix UnboundLocalError instead of proper error message (#221)

* fix UnboundLocalError: local variable 'platform' referenced before assignment
* add test to protect against regression

cibuildwheel/__main__.py

@@ -61,25 +61,30 @@ def main():
     args = parser.parse_args()
 
     detect_obsolete_options()
-
+    
     if args.platform != 'auto':
         platform = args.platform
     else:
         ci = strtobool(os.environ.get('CI', 'false')) or 'BITRISE_BUILD_NUMBER' in os.environ or 'AZURE_HTTP_USER_AGENT' in os.environ
-        if ci:
-            if sys.platform.startswith('linux'):
-                platform = 'linux'
-            elif sys.platform == 'darwin':
-                platform = 'macos'
-            elif sys.platform == 'win32':
-                platform = 'windows'
-        if platform is None:
+        if not ci:
             print('cibuildwheel: Unable to detect platform. cibuildwheel should run on your CI server, '
                   'Travis CI, AppVeyor, Azure Pipelines and CircleCI are supported. You can run on your '
                   'development machine or other CI providers using the --platform argument. Check --help '
                   'output for more information.',
                   file=sys.stderr)
             exit(2)
+        if sys.platform.startswith('linux'):
+            platform = 'linux'
+        elif sys.platform == 'darwin':
+            platform = 'macos'
+        elif sys.platform == 'win32':
+            platform = 'windows'
+        else:
+            print('cibuildwheel: Unable to detect platform from "sys.platform" in a CI environment. You can run '
+                  'cibuildwheel using the --platform argument. Check --help output for more information.',
+                  file=sys.stderr)
+            exit(2)
+
 
     output_dir = args.output_dir
     test_command = get_option_from_environment('CIBW_TEST_COMMAND', platform=platform)
@@ -174,7 +179,7 @@ def main():
     elif platform == 'macos':
         cibuildwheel.macos.build(**build_options)
     else:
-        raise Exception('Unsupported platform')
+        raise Exception('Unsupported platform: {}'.format(platform))
 
 
 def detect_obsolete_options():