[Bug] Error shadows checkArgumentType warning

[Iximiel]

The CI is failing on /regtest/secondarystructure/rt-coverage-antibetarmsd, see #1166
I found an error that may not be linked directly to that test failure:

The Error text is quite cryptic:

Caution

WARNING: type for input argument has not been specified

Because it does not specify the Action that generates the problem

The "warning" that raises the error is raised here:

/*** from tools/Keywords.cpp ***/
bool Keywords::checkArgumentType( const std::size_t& rank, const bool& hasderiv ) const {
  for(auto const& x : argument_types ) {
    if( rank==0 && x.second.find("scalar")!=std::string::npos ) return true;
    if( hasderiv && x.second.find("grid")!=std::string::npos ) return true;
    if( rank==1 && x.second.find("vector")!=std::string::npos ) return true;
    if( rank==2 && x.second.find("matrix")!=std::string::npos ) return true;
  }
  plumed_merror("WARNING: type for input argument has not been specified");
  return false;
}

and shadows the original intended WARNING message, which reports the name of the action that generates the WARNING:

/*** from core/ActionWithArguments.cpp ***/
void ActionWithArguments::interpretArgumentList(const std::vector<std::string>& c, const ActionSet& as, Action* readact, std::vector<Value*>&arg) {
  /*a lot of other code*/
  for(unsigned i=0; i<arg.size(); ++i) {
    if( !readact->keywords.checkArgumentType( arg[i]->getRank(), arg[i]->hasDerivatives() ) ){
     readact->warning("documentation for input type is not provided in " + readact->getName() );
    }
  }
}

I think the intended use was to transform the warning into an error
and that /regtest/secondarystructure/rt-coverage-antibetarmsd

I'm opening a PR with the ->warning changed to ->error as soon I verify some other things