Improve error message when variable is used but not defined

[sunxd3]

When a variable is referenced on the RHS, but not provided in data or appear on LHS, the compiler should error.

Previously, compile will error on these cases, but the error message can be cryptic -- usually in an array-out-of-bound type of errors.

This PR catches and reports these errors in a more graceful manner.

[coveralls]

Pull Request Test Coverage Report for Build 10900481812

Details

• 36 of 39 (92.31%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 83.059%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/compiler_pass.jl	35	38	92.11%
<!--	Total:	36	39	92.31%	-->

Totals
Change from base Build 10881242099:	0.2%
Covered Lines:	1466
Relevant Lines:	1765

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💛 - Coveralls