target: rewrite command 'write_memory' as COMMAND_HANDLER

target: rewrite command 'write_memory' as COMMAND_HANDLER

While there:
- drop the command name from the error messages;
- check the returned value from Jim_GetWide() to detect incorrect
  numeric values.

Change-Id: I399402ac11b6d459f1771e59e44210aef3e2a637
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8582
Tested-by: jenkins
Reviewed-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>