better error message on failure to open JSON file
better error message on failure to open JSON file

SymbiFlow/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr