bug#27143: [PATCH] gnu: build: Improve error reporting.

[Danny Milosavljevic]

* gnu/build/file-systems.scm (read-iso9660-primary-volume-descriptor):
Improve error reporting.
---
 gnu/build/file-systems.scm | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/gnu/build/file-systems.scm b/gnu/build/file-systems.scm
index 17c1182ce..bd6997adc 100644
--- a/gnu/build/file-systems.scm
+++ b/gnu/build/file-systems.scm
@@ -245,7 +245,10 @@ Trailing spaces are trimmed."
   "Find and read the first primary volume descriptor, starting at OFFSET.
    Return #f if not found."
   (let* ((sblock    (read-superblock device offset 2048 iso9660-superblock?))
-         (type-code (if sblock (array-ref sblock 0) 255)))
+         (type-code (if sblock
+                      (array-ref sblock 0)
+                      (error (format #f "Could not read ISO9660 primary
+volume descriptor from ~s" device)))))
     (match type-code
       (255 #f) ; Volume Descriptor Set Terminator.
       (1 sblock) ; Primary Volume Descriptor