The error message for an empty string as a directory name is unclear. This
condition is reproduced by configuring a project, selecting "Advanced Project
Options", clicking on "Use custom workspace", and not providing a "Directory"
field. The UI for this does not issue any warning at configuration time. The
only error occurs later, as a novice-unfriendly exception:

FATAL: Failed to mkdirs:
java.io.IOException: Failed to mkdirs:
at hudson.FilePath.mkdirs(FilePath.java:378)

The problem is that the failing directory name(s) is an empty string that
follows "Failed to mkdirs:". The code generating this error follows:
public void mkdirs() throws IOException, InterruptedException {
[...deleted...]
throw new IOException("Failed to mkdirs: "+remote);

A simple way to clarify this is to enhance the IOException to include quotes
around the directory names and be more explicit, e.g.

"Failed to create these directories: \"" + remote + "\""

(Where the quotes delimit the string, and make an empty string immediately obvious.)

This is a minor issue, and I only mention it because it requires a novice user
to have to download Hudson source code and review the exception details before
understanding what failed.

Thanks much.