Update src/stdlib_hashmap_wrappers.f90

Co-authored-by: Ian Giestas Pauli <iangiestaspauli@gmail.com>

Author: wclodius2

src/stdlib_hashmap_wrappers.f90

@@ -206,7 +206,7 @@ subroutine get_char_key( key, value )
             key_as_char = key_size
         case(2)
             if ( iand( key_size, 1_int64 ) > 0 ) then
-                stop 'KEY does not map to a character string.'
+                error stop 'Internal Error at stdlib_hashmaps: System uses 2 bytes per character, so key_size can't be an odd number'
             end if
             key_as_char = ishft( key_size, -1 )
         case(4)