fix(migration): improve runner migration and plugin installation feedback

src/langbot/pkg/plugin/errors.py

@@ -0,0 +1,24 @@
+"""Shared plugin operation errors without runtime or application imports."""
+
+
+class PluginRuntimeNotConnectedError(RuntimeError):
+    """Raised when plugin runtime operations are requested before connection."""
+
+
+class PluginInstallationFailedError(RuntimeError):
+    """Stable Runtime desired-state failure for one plugin installation."""
+
+    def __init__(
+        self,
+        installation_uuid: str,
+        error_code: str,
+        message: str,
+    ) -> None:
+        self.installation_uuid = installation_uuid
+        self.error_code = error_code
+        self.runtime_message = message
+        super().__init__(f'Plugin installation {installation_uuid} failed [{error_code}]: {message}')
+
+
+class MarketplacePluginVersionNotFoundError(ValueError):
+    """The requested release is not available from the marketplace."""