libfstools: handle gzip return value in block_volume_format

libfstools: handle gzip return value in block_volume_format

Fix the following compilation warning:
libfstools/common.c: In function 'block_volume_format':
libfstools/common.c:120:17: error: ignoring return value of 'system' declared with attribute 'warn_unused_result' [-Werror=unused-result]
  120 |                 system(str);
      |                 ^~~~~~~~~~~

Correctly handle return value from gzip and print an error on such case.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>