terrain file validation enhancement

[rnugent3]

If not all of the terrain files are in the appropriate directory, we hit an unhandled exception.

In this case, there is an HDF and a VRT in the terrain directory. The study is available at the below location. Attempt to compute stage-damage to replicate.

\\share.hec.usace.army.mil\USACE\Nugent\ForArden\Freeport Run 1-4-24