Clarify help message in LGE source extractor

scripts/LGE/extract-from-source.sh

@@ -9,7 +9,7 @@ else
 	SOURCE="${1}"
 	TARGET="${2}"
 	[ -d "${SOURCE}" ] || die "SOURCE_DIRECTORY must be a directory (containing one ALTO/LGE/T<T> directory with one ALTO-XML file per page)"
-	[ -d "${TARGET}" ] || die "TARGET_DIRECTORY must be a directory (where output will be generated)"
+	[ -d "${TARGET}" ] || die "TARGET_DIRECTORY must be a(n existing) directory (where output will be generated)"
 fi
 
 cd "${SOURCE}"