Warn user if z's datafile path is a directory.

Cause this means z will fill up the directory with temp files, and not
work.

README

@@ -120,7 +120,8 @@ ENVIRONMENT
 
 FILES
        Data  is  stored  in  $HOME/.z.  This  can be overridden by setting the
-       $_Z_DATA environment variable.
+       $_Z_DATA environment variable. When initialized, z will raise an  error
+       if this path is a directory, and not function correctly.
 
        A man page (z.1) is provided.
 

z.1

@@ -144,7 +144,8 @@ Directories must be full paths without trailing slashes.
 .SH
 FILES
 Data is stored in \fB$HOME/.z\fR. This can be overridden by setting the
-\fB$_Z_DATA\fR environment variable.
+\fB$_Z_DATA\fR environment variable. When initialized, \fBz\fR will raise an
+error if this path is a directory, and not function correctly.
 .P
 A man page (\fBz.1\fR) is provided.
 .SH

z.sh

@@ -27,6 +27,10 @@ case $- in
    *) echo 'ERROR: z.sh is meant to be sourced, not directly executed.'
 esac
 
+[ -d "${_Z_DATA:-$HOME/.z}" ] && {
+    echo "ERROR: z.sh's datafile (${_Z_DATA:-$HOME/.z}) is a directory."
+}
+
 _z() {
 
  local datafile="${_Z_DATA:-$HOME/.z}"