Adjust error message when computing [M/H] with custom zfracs

Close #515

Reported-by Claudia Reyes

Author: rjfarmer

chem/public/chem_def.f90

@@ -297,6 +297,7 @@ module chem_def
 
 
       ! identifiers for different Z fractions.
+      integer, parameter :: Custom_zfracs = 0
       integer, parameter :: AG89_zfracs = 1
       integer, parameter :: GN93_zfracs = 2
       integer, parameter :: GS98_zfracs = 3

chem/public/chem_lib.f90

@@ -706,6 +706,8 @@ real(dp) function chem_M_div_h(x,z,zfrac_choice) ! Returns [M/H]
             case(A09_Prz_zfracs)
                zsolar = A09_Prz_zsol
                ysolar = A09_Prz_ysol 
+            case(Custom_zfracs)
+               call mesa_error(__FILE__,__LINE__,"[M/H] not supported with custom zfracs.")
             case default
                call mesa_error(__FILE__,__LINE__,"Bad zfrac_choice")
          end select

star/private/adjust_xyz.f90

@@ -1013,7 +1013,7 @@ subroutine get_xa_for_standard_metals( &
                zfrac(:) = L09_element_zfrac(:)
             case (A09_Prz_zfracs)
                zfrac(:) = A09_Prz_zfrac(:)
-            case (0) ! use non-standard values given in controls
+            case (Custom_zfracs) ! use non-standard values given in controls
                zfrac(:) = 0
                zfrac(e_li) = s% z_fraction_li
                zfrac(e_be) = s% z_fraction_be