According to the help message, both --failonwarnings+ and --failonwarnings- should be valid.
But epubcheck 5.1.0 says about them, respectively:

Unrecognized argument: "--failonwarnings+"
EPUBCheck v5.1.0
...

and

Unrecognized argument: "--failonwarnings-"
EPUBCheck v5.1.0
...

respectively.