- Added some context to an error message - it's useful to know _which_
plugin failed when invoking the binary failed
- Replaced `json.Umarshal` with a `json.Decoder`, so that the
`UseNumber` function can be called, which prevents large integers from
being interpreted as `float64`s.
- Fixed a couple `log.Warn` calls that should've been `log.Warnf`
Signed-off-by: Dave Henderson <dhenderson@gmail.com>