Add comments to Jenkinsfile; limit Slack messages to errors
Add comments to Jenkinsfile; limit Slack messages to errors

Add comments for each stage of the Jenkinsfile build process and limit
Slack messages to failures.

aaronxsu/raster-foundry,raster-foundry/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,raster-foundry/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,azavea/raster-foundry,azavea/raster-foundry