Exact Error not appearing in case of poll scenarios

[Dray56]

Let's say I try to push an app and get this as an error

│ error while waiting for app multi-buildpack-app package to build: received state FAILED while waiting
│ for async process

It clearly gives me a generic error obtained during polling the app state and does not give the exact problem which occurred during the push which is of not much help to the end user. Better would be to capture the error details from the job/build(or any other associated resource if it is available) while pushing it and display it incase polling fails - something similar to this.

│ error while waiting for app multi-buildpack-app package to build: received state FAILED while waiting for async process
│ StagingError - Staging error: Start command not specified

[sneal]

Fixed by #450