Fix #326: missing include file now fatal
Fix #326: missing include file now fatal

The include file input tag now calls egsFatal if the file does not
exist, instead of egsWarning. This avoids the warning going unnoticed.
crcrewso/EGSnrc,MartinMartinov/EGSnrc,ftessier/EGSnrc,crcrewso/EGSnrc,marenaud/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,mchamberland/EGSnrc,mchamberland/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,ftessier/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,crcrewso/EGSnrc,nrc-cnrc/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,ftessier/EGSnrc,marenaud/EGSnrc,marenaud/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,ftessier/EGSnrc,crcrewso/EGSnrc,MartinMartinov/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,nrc-cnrc/EGSnrc,ftessier/EGSnrc,nrc-cnrc/EGSnrc,ftessier/EGSnrc,nrc-cnrc/EGSnrc