Put all exceptions in their dedicated module.

src/tscut/tools/discovery.py

@@ -8,10 +8,10 @@ from shutil import which
 
 from typeguard import typechecked
 
+from tscut.exceptions import MissingToolError
+
 logger = logging.getLogger(__name__)
 
-class MissingToolError(Exception):
-    pass
 
 @typechecked
 def check_required_tools() -> tuple[bool,dict[str,str]]: