Add bad_variant_access throw for unsupported architecture

Signed-off-by: Tomasz Michalak <tmichalak@antmicro.com>

lib/include/prjxray/xilinx/architectures.h

diff --git a/lib/include/prjxray/xilinx/architectures.h b/lib/include/prjxray/xilinx/architectures.h
index cdf992e..9459f7a 100644
--- a/lib/include/prjxray/xilinx/architectures.h
+++ b/lib/include/prjxray/xilinx/architectures.h
@@ -81,7 +81,7 @@
 		} else if (arch == "UltraScalePlus") {
 			return UltraScalePlus();
 		} else {
-			return Architecture::Container();
+			throw absl::bad_variant_access();
 		}
 	}
 };