make exception tracebacks easier to grok

[daira]

See #2116 for an example of a traceback involving three machines (the CLI, the gateway, and the storage server). The delimiters between parts of the traceback are not clear (and there are some very long lines which don't help). Make them clearer by adding markers at the start of each line.

[daira]

Note that this ticket is concerned primarily with readability to developers. There's a separate issue about how to present errors to users without making them overcomplicated, but regardless of that, there should still be a way for developers to get the full nested traceback.