Write error-level logs to stderr

[jpb]

Error log messages are written to stdout, which will get swallowed if piped into another command:

$ cat test.yaml
foo: !$ a

$ bin/iidy render test.yaml --format json | jq
parse error: Invalid numeric literal at line 1, column 6

Error is from jq ☝️

With this change:

$ bin/iidy render test.yaml --format json | jq
error Could not find "a" at a in Root.foo

Error is from iidy ☝️ (printed to stderr)