Make asset-write-date return -1 when one of the sources is missing.
diff --git a/asset.lisp b/asset.lisp
index 889bdff..43b1242 100644
--- a/asset.lisp
+++ b/asset.lisp
@@ -82,8 +82,11 @@
(ignore-errors (format stream " ~S" (asset-source-path asset)))))
(defmethod asset-write-date ((assets cons))
- (loop for a in assets
- maximize (file-write-date (asset-source-path a))))
+ (loop
+ for a in assets
+ for path = (asset-source-path a)
+ unless (probe-file path) do (return -1)
+ maximize (file-write-date path)))
(defmethod asset-sources% ((asset asset))
(list asset))