Added check for unsupported external source type.
Added check for unsupported external source type.

bhermanmit/cdash,shenqicang/openmc,keadyk/openmc_mg_prepush,keadyk/openmc_mg_prepush,lilulu/openmc,mit-crpg/openmc,lilulu/openmc,bhermanmit/openmc,wbinventor/openmc,samuelshaner/openmc,keadyk/openmc_mg_prepush,shikhar413/openmc,samuelshaner/openmc,liangjg/openmc,shikhar413/openmc,johnnyliu27/openmc,mjlong/openmc,nhorelik/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,paulromano/openmc,amandalund/openmc,smharper/openmc,johnnyliu27/openmc,lilulu/openmc,walshjon/openmc,bhermanmit/openmc,kellyrowland/openmc,samuelshaner/openmc,wbinventor/openmc,liangjg/openmc,wbinventor/openmc,walshjon/openmc,walshjon/openmc,mit-crpg/openmc,sxds/opemmc,walshjon/openmc,samuelshaner/openmc,wbinventor/openmc,mjlong/openmc,sxds/opemmc,johnnyliu27/openmc,shenqicang/openmc,shikhar413/openmc,liangjg/openmc,liangjg/openmc,shikhar413/openmc,amandalund/openmc,nhorelik/openmc,johnnyliu27/openmc,paulromano/openmc,amandalund/openmc,amandalund/openmc,kellyrowland/openmc,mit-crpg/openmc,smharper/openmc,mit-crpg/openmc,paulromano/openmc