Add exception handling for checks.d initialization. Fixes #427
Improve checks.d traceback passing for info command
Update agent check command to handle new config.load_check_directory signature
Remove need for checks.d CheckStatus.init_failed field
Rename checks.d CheckStatus.init_failed_exception to init_failed_error
for more consistent naming within checks
Merge pull request #500 from DataDog/427b-handle-check.d-exceptions

427b handle check.d exceptions