scr: update docs to document return value in Fortran failure case

[rhaas80]

This pull request does two things:

• adds a sentence to the SCR_Config docs explaining how Fortran reports that a parameter was not found (sets ierr to 1)
• move SCR_FAILURE into scr.h from scr_util.h

the second may be something to discuss though there is already code in eg scr_route_file that does return SCR_FAIULRE and not eg !SCR_SUCCESS so the value is already used (even though SCR_FAILURE == !SCR_SUCCESS anyway).