Correct example of exception override

[KLarpen]

Closes: https://github.com/metarhia/Docs/issues/21 Refs: https://github.com/metarhia/metacom/pull/486

When the PR https://github.com/metarhia/metacom/pull/486 will be landed — sentence about conversion of code 12345 and message to "Internal Server Error" with "code":500 becomes true. However conversion of exception message with code 404 (and all below known 599 range) still not happening automatically and there is no need to change that behavior. So this PR will correct example in Error-handling guidelines section at LAYERS page.