Use 'isa' checks for attribute validation

Use 'isa' checks for attribute validation

To maintain compatibility with the existing set-error-and-return-undef
API, we must jump through some hoops:

  1) Throw an object that won't be mangled by Moo's isa error prefixing
  2) Wrap things that might throw that to set ->error and return undef
  3) Store errors in the class when there is no object (i.e. ->new)