previous behavior was to log an error; this is fine in the general
case where no `spark.metrics.conf` parameter was specified, in which
case a default `metrics.properties` is looked for, and the execption
logged and suppressed if it doesn't exist.

if the user has purposefully specified a metrics.conf file, however,
it makes more sense to show them an error when said file doesn't
exist.

Author: Ryan Williams <ryan.blake.williams@gmail.com>

Closes #4571 from ryan-williams/metrics and squashes the following commits:

5bccb14 [Ryan Williams] private-ize some MetricsConfig members
08ff998 [Ryan Williams] rename METRICS_CONF: DEFAULT_METRICS_CONF_FILENAME
f4d7fab [Ryan Williams] fix tests
ad24b0e [Ryan Williams] add "metrics.properties" to .rat-excludes
94e810b [Ryan Williams] throw if nonexistent Sink class is specified
31d2c30 [Ryan Williams] metrics code review feedback
56287db [Ryan Williams] throw if nonexistent metrics config file provided