bug#36373: failed copy-file should print path

[Robert Vollmert]

I just ran into this error while trying to build a go package. There’s no
obvious way to figure out which file caused “permission denied”. I’d like
to both see the non-truncated filename in the backtrace, as well as in
the error message below.

Backtrace:
          10 (primitive-load "/gnu/store/zgwm1vv4vb5i91iphzs8wdqwpxr…")
In ice-9/eval.scm:
   191:35  9 (_ _)
In srfi/srfi-1.scm:
   863:16  8 (every1 #<procedure 65a360 at /gnu/store/zmc0hcmdfg5n4…> …)
In 
/gnu/store/zmc0hcmdfg5n4kl32vcla4cg9c9bspfg-module-import/guix/build/gnu-build-system.scm:
   799:28  7 (_ _)
In 
/gnu/store/zmc0hcmdfg5n4kl32vcla4cg9c9bspfg-module-import/guix/build/go-build-system.scm:
    187:8  6 (unpack #:source _ #:import-path _ #:unpack-path _)
In ice-9/ftw.scm:
   452:32  5 (loop _ _ #(2050 670124 16749 19 0 0 0 4096 # 1 # # 8 …) …)
In srfi/srfi-1.scm:
   466:18  4 (fold #<procedure 7a4d80 at ice-9/ftw.scm:452:38 (subd…> …)
In unknown file:
           3 (_ #<procedure 870c60 at ice-9/ftw.scm:454:44 ()> #<pr…> …)
In ice-9/ftw.scm:
   482:39  2 (loop _ _ #(2050 1721126 16749 2 0 0 0 4096 # 1 # # 8 …) …)
In 
/gnu/store/zmc0hcmdfg5n4kl32vcla4cg9c9bspfg-module-import/guix/build/utils.scm:
   312:27  1 (_ "/gnu/store/69hmclpsln83ldh9c94c0iygj4ajiyrl-go-x-t…" …)
In unknown file:
           0 (copy-file "/gnu/store/69hmclpsln83ldh9c94c0iygj4ajiyr…" …)

ERROR: In procedure copy-file:
In procedure copy-file: Permission denied