Error handling

What happens with the build if a step in the drone.yml produces an error?

It should probably handle errors differently depending on the build step: setup, processing, output handling, etc. For example, if a user specifies some files to be pushed back on success but some of the files don't exist, it should probably warn that some of the specified files don't exist but it shouldn't fail completely (or at least, the files that **do** exist should be pushed).

The issue names drone.yml but no source files or tests; start by tracing how its setup, processing, and output steps handle errors. Map the current behavior for missing output files and other step failures, then define completion as documented, consistent handling for each step, including warnings and any allowed partial success.

Written by the indexing model from the issue text.