fix: show clear error message when mounting FUSE without root

[XciD]

Summary

• Detect PermissionDenied error from fuser::Session::new() and display a helpful message with the exact sudo command to re-run, instead of a raw OS error.

Closes #10

[github-actions]

Benchmark Results

============================================================
  Benchmark — 50MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                    219.0 MB/s     253.4 MB/s
  Sequential re-read                1902.0 MB/s    1852.6 MB/s
  Range read (1MB@25MB)                0.5 ms         0.3 ms
  Random reads (100x4KB avg)           0.0 ms         0.0 ms
  Sequential write (FUSE)           1182.3 MB/s
  Close latency (CAS+Hub)            0.135 s
  Write end-to-end                   282.0 MB/s
  Dedup write                       1062.5 MB/s
  Dedup close latency                0.435 s
  Dedup end-to-end                   103.8 MB/s
============================================================
============================================================
  Benchmark — 200MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                    948.8 MB/s     263.0 MB/s
  Sequential re-read                1883.0 MB/s    2093.4 MB/s
  Range read (1MB@25MB)                0.3 ms         0.3 ms
  Random reads (100x4KB avg)           0.0 ms         0.0 ms
  Sequential write (FUSE)            944.5 MB/s
  Close latency (CAS+Hub)            0.769 s
  Write end-to-end                   203.9 MB/s
  Dedup write                       1091.8 MB/s
  Dedup close latency                0.190 s
  Dedup end-to-end                   535.8 MB/s
============================================================
============================================================
  Benchmark — 500MB
------------------------------------------------------------
  Metric                                 FUSE          NFS
  ------------------------------ ------------ ------------
  Sequential read                   1485.6 MB/s     499.4 MB/s
  Sequential re-read                1943.7 MB/s    2148.9 MB/s
  Range read (1MB@25MB)                0.3 ms         0.3 ms
  Random reads (100x4KB avg)           0.0 ms         0.0 ms
  Sequential write (FUSE)           1129.2 MB/s
  Close latency (CAS+Hub)            0.602 s
  Write end-to-end                   478.4 MB/s
  Dedup write                        963.9 MB/s
  Dedup close latency                0.166 s
  Dedup end-to-end                   730.4 MB/s
============================================================
============================================================
  fio Benchmark Results
------------------------------------------------------------
  Job                        FUSE MB/s   NFS MB/s  FUSE IOPS   NFS IOPS
  ------------------------- ---------- ---------- ---------- ----------
  seq-read-100M                  458.7      214.1                      
  seq-reread-100M               2040.8      365.0                      
  rand-read-4k-100M                0.1        0.1         30         27
  seq-read-5x10M                 746.3      746.3                      
  rand-read-10x1M                133.5       43.7      34186      11184
  Random Read Latency           FUSE avg      NFS avg
  ------------------------- ------------ ------------
  rand-read-4k-100M           33760.1 us   36813.8 us
  rand-read-10x1M                28.5 us      88.3 us
============================================================