Better error messages on config load failure

[greenhat]

Close #305

[coveralls]

Pull Request Test Coverage Report for Build 5595507511

• 2 of 15 (13.33%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 54.538%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/main.rs	0	3	0.0%
core/src/oracle_config.rs	2	7	28.57%
core/src/pool_config.rs	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/src/oracle_config.rs	1	13.95%

Totals
Change from base Build 5551996418:	-0.07%
Covered Lines:	2061
Relevant Lines:	3779

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💛 - Coveralls