B64enc and B64dec can be replaced by base64.RawURLEncoding.
Thumbprint is now implemented in go-jose, and we have the relevant version
imported already, so we can use that.
SyntaxError isn't used anywhere and can be deleted.
This allows us to return a for-users ProblemDetails from services that
the WFE calls (or services that those services call). Doing so means we
can give better errors to the WFE once we teach the WFE how to pass
along ProblemDetails itself.
This is for #1153 and a few other tickets in the error-messages label.
Fixes#1160
Jacob Hoffman-Andrews
Jeff Hodges
Roland Shoemaker