enable rich tracebacks

console, error_console are now app attributes.

src/vban_cli/error.py

@@ -0,0 +1,14 @@
+class VbanCLIError(Exception):
+    """Base class for exceptions in this module."""
+
+    def __init__(self, message: str, code: int = 1):
+        super().__init__(message)
+        self.code = code
+
+
+class VbanCLIConnectionError(VbanCLIError):
+    """Exception raised for connection errors."""
+
+
+class VbanCLIValidationError(VbanCLIError):
+    """Exception raised for validation errors."""