Idea: Allow custom validation errors

[andy128k]

Current implementation of an VaidationError stores a String. This forces users to convert errors to a string makes analyzing of custom errors hard.

ValidationError's definition can be changed from ValdationError(String) to ValdationError(Box). The drawback of such change is that std::error::Error is not supported in no_std.

[TedDriggs]

This is where a custom error type would be useful; the way we emit the error in the validation case would allow for that function to return impl Into<BuildFnError>, and allowing the caller to tell us which error type to use would solve that. See #191 for this.

[andy128k]

#194 Solves this issue.