Switch to %-style log formatting

We should let Python's logging module lazily interpolate the string.
This is generally recommended as the way to write log messages (over
eagerly formatting with either .format() or f-strings). This has two
benefits:

1) The 'processing' required to calculate the value of the variable
being interpolated doesn't need to happen unless the log message is
actually being emitted.
2) Sentry/etc can group together error log lines based on the base
unformatted message. If we eagerly interpolate, each log message will be
unique and so they don't get grouped together.