master: optimize _return()

Be optimistic about file layout by not checking file existence (which
introduces additional `stat(2)` calls) but try to create dir and handle
errors in case of failure.

This reduces number of stat calls by 3 per minion per job.

From:
```
    stat("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a", {st_mode=S_IFDIR|0755, st_size=86016, ...}) = 0 <0.000011>
    stat("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a/nocache", 0x7fff278982a0) = -1 ENOENT (No such file or directory) <0.000010>
    stat("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a/host000000.linkedin.com", 0x7fff278982a0) = -1 ENOENT (No such file or directory) <0.000018>
    stat("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a", {st_mode=S_IFDIR|0755, st_size=86016, ...}) = 0 <0.000011>
    mkdir("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a/host000000.linkedin.com", 0777) = 0 <0.000035>
```

To:
```
    stat("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a/nocache", 0x7fff278982a0) = -1 ENOENT (No such file or directory) <0.000010>
    mkdir("/salt/cache/master/jobs/dd/557c790ce8f7f15dbaa679937f130a/host000000.linkedin.com", 0777) = 0 <0.000035>
```