Detect obviously erroneous "memory_limit" configurations

Summary: See comment. ALSO: THIS DIFF IS AMAZING.
Test Plan: Changed my memory limit to something absurd, got yelled at.
Reviewed By: mroch
Reviewers: colmdoyle, jungejason, aran, tuomaspelkonen, codeblock, tomo, mroch,
hsb
CC: aran, mroch, epriestley
Differential Revision: 657