Change error type for redirect parsing errors (#1649)

* Change error type for redirect parsing errors

This changes the error type returned when the proxy fails to parse the
redirect target to be a 400 error instead of a 500 error.

As far as I can tell, the only way that this can fail is a failure to
parse the properties of the request to identity the redirect target.
This indicates that the user has sent a malformed request, and so should
result in a 400 rather than a 500.

I've added a test to exercise this, based on a real work example.

* Update changelog

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

oauthproxy_test.go

@@ -678,6 +678,17 @@ func TestSignInPageIncludesTargetRedirect(t *testing.T) {
 	}
 }
 
+func TestSignInPageInvalidQueryStringReturnsBadRequest(t *testing.T) {
+	sipTest, err := NewSignInPageTest(true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	const endpoint = "/?q=%va"
+
+	code, _ := sipTest.GetEndpoint(endpoint)
+	assert.Equal(t, 400, code)
+}
+
 func TestSignInPageDirectAccessRedirectsToRoot(t *testing.T) {
 	sipTest, err := NewSignInPageTest(false)
 	if err != nil {