code: public <T extends Throwable> RestHandlerBuilder addExceptionMapper(Class<T> exceptionClass, ExceptionMapper<T> exceptionMapper) {
          this.exceptionMappers.put(exceptionClass, exceptionMapper);
          return this;
      }
docs: <p>Adds a mapper that converts an exception to a response.</p>
  <p>For example, you may create a custom exception such as a ValidationException that you throw from your
  jax-rs methods. A mapper for this exception type could return a Response with a 400 code and a custom
  validation error message.</p>
  
  @param <T>             The exception type that the mapper can handle
  @param exceptionClass  The type of exception to map.
  @param exceptionMapper A function that creates a {@link javax.ws.rs.core.Response} suitable for the exception.
  @return Returns this builder.