Fix error conditions when warnings occur in validation output

[ericglau]

When running validations via upgrades core CLI:

• success is indicated by exit code 0 and "SUCCESS"
• failures are indicated by exit code 1 and "FAILURE"
• warnings are written to stderr. This is intentional, and allows the warnings to be separate from validation results output (which has extraneous information such as which contracts passed, which we want to ignore if validations succeded)

Previously, if exit code was 1 (validations failed) and there is content in stderr, we would revert with stderr as the reason. This is misleading because stderr only contains the warnings, and not the actual validation results.

Instead, we should log all warnings (from stderr) if the CLI finished with stdout containing "SUCCESS" or "FAILURE", and then report validation results appropriately.

If stdout not have "SUCCESS" or "FAILURE", this indicates a problem with running the CLI itself.

Fixes #83
Fixes OpenZeppelin/openzeppelin-upgrades#1117