Filtering with invalid condition shouldn't crash the server

[jvshahid]

Currently running the following query will crash the server. This is related to #402

select * from foo where a > 0.1s`

In this example the user was storing time durations in a and tried to run a query that returns points with a greater than 0.1 seconds. This is caused the server to panic and die. Just for completeness, this query cannot be supported since the precision of the column a cannot be determined, it can be milliseconds, microseconds or nanoseconds.