Add Project Validation - Pre Execution

[eyal-mor]

When executing commands, the CLI may require some files to be available, top examples:

1. main.pipeline.jsonnet.
2. cleanup.pipeline.jsonnet.

These files should be validated pre-execution and have a clear error message requiring them. E.g.:

no such file: `main.pipeline.jsonnet`
`shore render` can only be invoked if `main.pipeline.jsonnet` exists.
  Please validate `main.pipeline.jsonnet` exist in the root of the project.

no such file: `cleanup.pipeline.jsonnet`
`shore render` can only be invoked if `cleanup.pipeline.jsonnet` exists.
  Please validate `cleanup.pipeline.jsonnet` exist in the root of the project.