code: public ConfigParseOptions setAllowMissing(boolean allowMissing) {
          if (this.allowMissing == allowMissing)
              return this;
          else
              return new ConfigParseOptions(this.syntax, this.originDescription, allowMissing,
                      this.includer, this.classLoader);
      }
docs: Set to false to throw an exception if the item being parsed (for example
  a file) is missing. Set to true to just return an empty document in that
  case. Note that this setting applies on only to fetching the root document,
  it has no effect on any nested includes.
  
  @param allowMissing true to silently ignore missing item
  @return options with the "allow missing" flag set