fix [object Object] in invalid decorator warning

when there's an invalid decorator, you get a warning like this:

```
[warning] INVALID decoration from extension '[object Object]': Error: The decoration is empty
```

as is done elsewhere in this file, should use `extensionId.identifier.value` to get a string

Author: mroch

src/vs/workbench/api/common/extHostDecorations.ts

@@ -106,7 +106,7 @@ export class ExtHostDecorations implements ExtHostDecorationsShape {
 					}
 					result[id] = <DecorationData>[data.propagate, data.tooltip, data.badge, data.color];
 				} catch (e) {
-					this._logService.warn(`INVALID decoration from extension '${extensionId}': ${e}`);
+					this._logService.warn(`INVALID decoration from extension '${extensionId.identifier.value}': ${e}`);
 				}
 			} catch (err) {
 				this._logService.error(err);