gdb/breakpoint: use warning function instead of gdb_printf

Noticed that in breakpoint.c, in one place, we do this:

  gdb_printf (_("warning: Error removing "
                "breakpoint %d\n"),
                old_loc->owner->number);

Instead of using the `warning` function.  There are a number of
differences between using gdb_printf like this and calling `warning`,
the main one is probably that real warnings are sent to gdb_stderr,
while the above gdb_printf call will go to gdb_stdout.

In this commit I:

  1. Change to call `warning`, we can drop the "warning: " prefix from
  the string in breakpoint.c,

  2. Update the warning text, I now start with a lower case 'e', which
  I believe is the GDB style for warnings,

  3. And I have included the address of the bp_location in the warning
  messsage,

  4. Finally, I update all the tests (2) that include this error
  message.

Reviewed-By: Tom Tromey <tom@tromey.com>