Add latex warning for undefined queries

[rebeccamccabe]

Redefines the final else block to include a warning, keeping the ?? output as before:

\else%
  \PackageWarning{json2latex}{Undefined query `#1'}%
  \def\resultsRE@out{%
??}%

Example latex log entry:

Package json2latex Warning: Undefined query `numLinConstraints' on input line 236.