Gracefully handle missing Spinnaker application

## Summary

Spinnaker will accept a request to create a pipeline, even when the pipeline's application doesn't exist. The pipeline is inaccessible until the application is created. `shore` could catch this misconfiguration, and raise an error, informing the user to create the application first.

## Motivation

There is some confusion caused when shore says it created a pipeline, but the user then isn't able to find the pipeline in Spinnaker until the application is created. For someone inexperienced with Spinnaker, it isn't obvious that that the reason they can't find their pipeline is because the application doesn't exist, and not because shore failed to save the pipeline. Furthermore, once the application is created, it can be confusing why there's suddenly pipelines that were created before the application.

## Describe alternatives you've considered

shore could create the Spinnaker application, however this would require collecting some metadata used by the application (i.e. creator email).

## Additional context

Start by tracing shore's pipeline-creation flow and how it communicates with Spinnaker. Determine where the application associated with a pipeline can be checked before creation. Done means a missing application produces an actionable error telling the user to create the application first, rather than reporting a successful pipeline creation.

Written by the indexing model from the issue text.