partial json ticker causes "ungraceful" termination

[mr-highball]

need better validation on ticker files when pre-loading data. when line is detected that "appears" to be valid actually isn't, non-user friendly errors occur and force the user to remove the malformed line.

acceptance criteria

• generate warning in log when bad line occurs, but move on gracefully
• {"value": 563, sample in last line is partial json but should be skipped since it is invalid