error logs and verbose mode

[dherman]

We need an RFC spelling out how error logging should work. A first sketch:

Error log

• Whenever errors occur, they should be stored in a log file somewhere within the user's Notion directory.
• The main log file should put the most useful information at the top -- probably the error message with some combination of that error's stack trace and/or the root cause error's stack trace.
• There should be extremely clear separators between errors in the cause chain.
• The log file should contain the whole cause chain, with details that are less likely to be useful pushed down lower than the most useful info.

Log rolling

• A small number of older error logs should be automatically saved in a subdirectory of the user's Notion directory, maybe just 10 with .1.log through .9.log suffixes.
• Perhaps the number of archived logs could be configurable via the config file, but this seems lower priority.

Some possible other commands

• notion logfile -- print out the path to the log file for easy piping to other commands
• notion log -- print out a nicely formatted log of the last generated error log, piped to a paginator

Verbose mode

• When Notion is run in verbose mode, the CLI output should be similar to notion log.

[charlespierce]

While there is still room to iterate on the specific implementation, we do have error logs (#31) and verbose mode now.