Generate a useful error if config doesn't exits

Signed-off-by: Eric F Crist <ecrist@secure-computing.net>

ChangeLog

@@ -8,6 +8,7 @@ Easy-RSA 3 ChangeLog
    * Workaround for ash and no set -o echo (#319)
    * Shore up windows testing framework (#314)
    * Provide upgrade mechanism for older versions of EasyRSA (#349)
+   * Add support for KDC certificates (#322)
 
 3.0.6 (2019-02-01)
    * Certificates that are revoked now move to a revoked subdirectory (#63)

easyrsa3/easyrsa

@@ -1600,7 +1600,11 @@ vars_setup() {
 	pki_vars="${EASYRSA_PKI:-$PWD/pki}/vars"
 
 	# command-line path:
-	if [ -f "$EASYRSA_VARS_FILE" ]; then
+	if [ ! -z "$EASYRSA_VARS_FILE" ]; then
+	  if [ ! -f "$EASYRSA_VARS_FILE" ]; then
+		  # If the --vars option does not point to a file, show helpful error.
+			die "The file '$EASYRSA_VARS_FILE' was not found."
+		fi
 		vars="$EASYRSA_VARS_FILE"
 	# PKI location, if present:
 	elif [ -f "$pki_vars" ]; then