Check return status of commands that can fail

[storborg]

[storborg]

Here's some thoughts on how to approach this:

• Make dram creator scripts essentially just a shell script (or arbitrary executable) that gets passed the dram name, path, and any additional arguments passed on the command line.
• Include a bunch of creator scripts in the distribution, but also allow them to be overridden (by name) or supplemented by additional dram creators in e.g. .dram/creators.
• Also allow the argument to the -t option to be a URL: if one is supplied, download it and run it like a normal executable.