fix: Avoid logging for missing backends

As they may also be part of a backend search loop

Author: alecandido

src/qibo/backends/__init__.py

@@ -231,8 +231,7 @@ def construct_backend(backend, **kwargs) -> Backend:
         # pylint: disable=unsupported-membership-test
         if provider not in e.msg:
             raise e
-        raise_error(
-            MissingBackend,
+        raise MissingBackend(
             f"The '{backend}' backends' provider is not available. Check that a Python "
             f"package named '{provider}' is installed, and it is exposing valid Qibo "
             "backends.",