fix: diagnose, but not throw for unicode error
fix: diagnose, but not throw for unicode error

in 2.0.x and 2.1.0, README says "it shows warning" for invalid unicode
codepoints. So far, this library just show an error message in stderr
for this case. It is not good to change the behavior fatal in the next
minor release, 2.1.1, that includes patches and improved error msgs.
I will make it throw syntax_error after 2.2.0 for invalid unicode
codepoints. For now, I will keep it to be "warning".

ToruNiina/toml11