Improve error handling in buildcache downloads (#35568)

The checksum exception was not detailed enough and not reraised when using cache only, resulting in useless error messages.

Now it dumps the file path, expected
hash, computed hash, and the downloaded file summary.

lib/spack/llnl/util/filesystem.py

@@ -2630,3 +2630,28 @@ def temporary_dir(
             yield tmp_dir
     finally:
         remove_directory_contents(tmp_dir)
+
+
+def filesummary(path, print_bytes=16) -> Tuple[int, bytes]:
+    """Create a small summary of the given file. Does not error
+    when file does not exist.
+
+    Args:
+        print_bytes (int): Number of bytes to print from start/end of file
+
+    Returns:
+        Tuple of size and byte string containing first n .. last n bytes.
+        Size is 0 if file cannot be read."""
+    try:
+        n = print_bytes
+        with open(path, "rb") as f:
+            size = os.fstat(f.fileno()).st_size
+            if size <= 2 * n:
+                short_contents = f.read(2 * n)
+            else:
+                short_contents = f.read(n)
+                f.seek(-n, 2)
+                short_contents += b"..." + f.read(n)
+        return size, short_contents
+    except OSError:
+        return 0, b""