fix: Make error type more specific

src/qibo/backends/__init__.py

@@ -14,6 +14,10 @@
 QIBO_NATIVE_BACKENDS = ("numpy", "tensorflow", "pytorch", "qulacs")
 
 
+class MissingBackend(ValueError):
+    """Impossible to locate backend provider package."""
+
+
 class MetaBackend:
     """Meta-backend class which takes care of loading the qibo backends."""
 
@@ -89,7 +93,7 @@ def __new__(cls):
                 try:
                     cls._instance = construct_backend(**kwargs)
                     break
-                except (ModuleNotFoundError, ImportError):
+                except MissingBackend:
                     pass
 
         if cls._instance is None:  # pragma: no cover
@@ -228,7 +232,7 @@ def construct_backend(backend, **kwargs) -> Backend:
         if provider not in e.msg:
             raise e
         raise_error(
-            ImportError,
+            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.",