pmb.core.arch: Handle unsupported arches in from_machine_type() better (MR 2498)

Closes https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/issues/2501

pmb/core/arch.py

@@ -61,6 +61,8 @@ class Arch(enum.Enum):
                 return Arch.armhf
             case "armv7l" | "armv8l":
                 return Arch.armv7
+            case _:
+                raise ValueError(f"Unsupported machine type '{machine_type}'")
 
     @staticmethod
     def native() -> "Arch":

pmb/core/test_arch.py

@@ -77,3 +77,7 @@ def test_invalid_arches():
     with pytest.raises(TypeError) as excinfo:
         "bap" / Arch.aarch64
     assert "unsupported operand type(s) for /: 'str' and 'Arch'" in str(excinfo.value)
+
+    with pytest.raises(ValueError) as excinfo:
+        Arch.from_machine_type("invalid")
+    assert "Unsupported machine type 'invalid'" in str(excinfo.value)