keep previous exception when transforming to dav exception

[icewind1991]

Hopefully makes for easier debugging

[rullzer]

Yeah lets backport. So we have info when somebody runs into something.

[codecov]

Codecov Report

Merging #8224 into master will decrease coverage by 1.01%.
The diff coverage is 40%.

@@ Coverage Diff @@## master #8224 +/- ##
============================================
- Coverage 52.75% 51.73% -1.02% - Complexity 24062 25366 +1304 
============================================
Files 1506 1599 +93 Lines 90305 95064 +4759 Branches 1376 1376 ============================================
+ Hits 47643 49185 +1542 - Misses 42662 45879 +3217

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/Connector/Sabre/Directory.php	65.6% <0%> (ø)	65 <0> (ø)	⬇️
.../dav/lib/Connector/Sabre/Exception/InvalidPath.php	81.81% <100%> (ø)	3 <1> (ø)	⬇️
lib/private/Server.php	83.18% <0%> (-0.1%)	282% <0%> (ø)
...pps/files_external/lib/Lib/DefinitionParameter.php	83.01% <0%> (ø)	23% <0%> (?)
apps/files_external/lib/Lib/IdentifierTrait.php	63.63% <0%> (ø)	8% <0%> (?)
apps/files_external/lib/Lib/DependencyTrait.php	0% <0%> (ø)	1% <0%> (?)
...iles_external/lib/Lib/Auth/OpenStack/Rackspace.php	0% <0%> (ø)	1% <0%> (?)
apps/files_external/lib/Lib/Backend/SFTP_Key.php	0% <0%> (ø)	1% <0%> (?)
...iles_external/lib/Lib/Auth/OpenStack/OpenStack.php	0% <0%> (ø)	1% <0%> (?)
...ernal/lib/Lib/Auth/Password/SessionCredentials.php	42.85% <0%> (ø)	4% <0%> (?)
... and 88 more