sysexits.h defines standard exit codes:

 EX_OK           0       /* successful termination */
 EX__BASE        64      /* base value for error messages */
 EX_USAGE        64      /* command line usage error */
 EX_DATAERR      65      /* data format error */
 EX_NOINPUT      66      /* cannot open input */    
 EX_NOUSER       67      /* addressee unknown */    
 EX_NOHOST       68      /* host name unknown */
 EX_UNAVAILABLE  69      /* service unavailable */
 EX_SOFTWARE     70      /* internal software error */
 EX_OSERR        71      /* system error (e.g., can't fork) */
 EX_OSFILE       72      /* critical OS file missing */
 EX_CANTCREAT    73      /* can't create (user) output file */
 EX_IOERR        74      /* input/output error */
 EX_TEMPFAIL     75      /* temp failure; user is invited to retry */
 EX_PROTOCOL     76      /* remote error in protocol */
 EX_NOPERM       77      /* permission denied */
 EX_CONFIG       78      /* configuration error */

Whether or not it is useful to stick to this convention is unclear.