mount: tell why FUSE support is unavailable

[ThomasWaldmann]

Fixes the actionable part of #8657 (see also #8388): borg mount only said no FUSE support without any hint at the cause, so users could not tell "not installed" from "installed, but broken" (e.g. a pyfuse3 wheel linked against a libfuse soname that no longer exists).

• fuse_impl: record the import error of each FUSE implementation that failed to load in a new fuse_import_errors dict. Also catch non-ImportError failures: mfusepy raises OSError at module import time if libfuse is missing (and AttributeError/NotImplementedError in other cases). Since mfusepy is the first default implementation, that previously crashed borg mount — and sysinfo() in exception handlers — with a raw traceback on any host without libfuse.

• borg mount: show the recorded errors:

  Runtime error: borg mount not available: no FUSE support, BORG_FUSE_IMPL=mfusepy,pyfuse3,llfuse.
  import of mfusepy failed: Unable to find libfuse
  import of pyfuse3 failed: libfuse3.so.3: cannot open shared object file: No such file or directory
  import of llfuse failed: No module named 'llfuse'
  

• sysinfo: it reported fuse: None when mfusepy was the active implementation (it only looked at llfuse). Now it reports the active module (mfusepy has no __version__, so the version is best-effort) and includes the recorded import errors when no implementation loaded — so the debug output users paste into issue reports shows the cause directly.

• tests: new test_fuse_import_errors_recorded covering the OSError case and the error recording.

• docs: note in the Standalone Binary section that third-party binaries might lack FUSE support and that OS-level FUSE is needed.

A 1.4-maint backport of the diagnostic part would be easy if wanted (1.4 only has pyfuse3/llfuse, so the OSError hazard does not exist there).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.00%. Comparing base (7babd39) to head (2927349).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/mount_cmds.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10159      +/-   ##
==========================================
- Coverage   87.00%   87.00%   -0.01%     
==========================================
  Files         101      101              
  Lines       17996    18002       +6     
  Branches     2738     2738              
==========================================
+ Hits        15657    15662       +5     
- Misses       1635     1637       +2     
+ Partials      704      703       -1     

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