Expand error message when OBC files are missing (#851)

* Expand error message when OBC files are missing

* Add error to check if file exists.

Add a check if the OBC file exists and give a fatal error if not.

* Fix typo

---------

Co-authored-by: Theresa Morrison <Theresa.Morrison@gaea61.ncrc.gov>

Author: theresa-cordero

src/core/MOM_open_boundary.F90

@@ -19,7 +19,7 @@ module MOM_open_boundary
 use MOM_interface_heights,    only : thickness_to_dz
 use MOM_interpolate,          only : init_external_field, time_interp_external, time_interp_external_init
 use MOM_interpolate,          only : external_field
-use MOM_io,                   only : slasher, field_size, SINGLE_FILE
+use MOM_io,                   only : slasher, field_size, file_exists, SINGLE_FILE
 use MOM_io,                   only : vardesc, query_vardesc, var_desc
 use MOM_obsolete_params,      only : obsolete_logical, obsolete_int, obsolete_real, obsolete_char
 use MOM_regridding,           only : regridding_CS
@@ -913,6 +913,8 @@ subroutine initialize_segment_data(G, GV, US, OBC, PF)
         fieldname = trim(fieldname)//trim(suffix)
         call field_size(filename,fieldname,siz,no_domain=.true.)
 !       if (siz(4) == 1) segment%values_needed = .false.
+        if (.not.file_exists(filename)) &
+          call MOM_error(FATAL," Unable to open OBC file " // trim(filename))
         if (segment%on_pe) then
           if (OBC%brushcutter_mode .and. (modulo(siz(1),2) == 0 .or. modulo(siz(2),2) == 0)) then
             write(mesg,'("Brushcutter mode sizes ", I6, I6)') siz(1), siz(2)