fix UnboundLocalError instead of proper error message (#221)

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

unit_test/platform_test.py

@@ -0,0 +1,39 @@
+import sys
+import os
+
+import pytest
+
+from cibuildwheel.__main__ import main
+
+
+def test_unknown_platform_non_ci(monkeypatch, capsys):
+    monkeypatch.setattr(os, 'environ', {})
+    monkeypatch.setattr(sys, "argv", ["python", "."])
+    with pytest.raises(SystemExit) as exit:
+        main()
+    assert exit.value.code == 2
+    _, err = capsys.readouterr()
+    assert 'cibuildwheel: Unable to detect platform.' in err
+    assert "cibuildwheel should run on your CI server" in err
+
+
+def test_unknown_platform_on_ci(monkeypatch, capsys):
+    monkeypatch.setattr(os, 'environ', {"CI": "true"})
+    monkeypatch.setattr(sys, "argv", ["python", "."])
+
+    monkeypatch.setattr(sys, "platform", "Something")
+
+    with pytest.raises(SystemExit) as exit:
+        main()
+    _, err = capsys.readouterr()
+    assert exit.value.code == 2
+    assert 'cibuildwheel: Unable to detect platform from "sys.platform"' in err
+
+
+def test_unknown_platform(monkeypatch):
+    monkeypatch.setattr(os, 'environ', {"CIBW_PLATFORM": "Something"})
+    monkeypatch.setattr(sys, "argv", ["python", "."])
+
+    with pytest.raises(Exception) as exc:
+        main()
+    assert exc.value.args[0] == 'Unsupported platform: Something'