add verbosity to error message
add verbosity to test case 531
FIX: unrecognized directory replacement
add descriptive comment to fixed code
Merge pull request #416 from reneme/fix-test_531

FIX: Replacing symlink/regular file by directory caused abort

Replacing a symlink/regular file in a repository by a directory might caused cvmfs_server sync to abort with '<filename>' has an unsupported file type!. This was partially fixed in Pull Request #374 but test case 531 failed again now.

When replacing a regular file or symlink by a directory and putting files in this newly created directory it will try to stat those files in the read-only branch. This leads to ENOTDIR. The code only checked for ENOENT though, and failed.