Raw exception shown when files do not exist

[kit-ty-kate]

$ cat packages/test/test.1/opam
opam-version: "2.0"
extra-files: ["does-not-exist" "md5=fa3f5d536909bb625ae9f77392261a3f"]
$ opam install test
The following actions will be performed:
=== recompile 1 package
  ↻ test 1

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>

#=== ERROR while compiling test.1 =============================================#
Sys_error("/home/kit_ty_kate/.opam/repo/tmp/packages/test/test.1/files/does-not-exist: No such file or directory")


<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build test 1
└─ 
╶─ No changes have been performed

$ cat packages/test/test.1/opam
opam-version: "2.0"
patches: "does-not-exist"
$ opam install test
The following actions will be performed:
=== recompile 1 package
  ↻ test 1

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ERROR] Patch file "/home/kit_ty_kate/.opam/5.3/.opam-switch/build/test.1/does-not-exist" not found.

#=== ERROR while compiling test.1 =============================================#
Not_found


<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build test 1
└─ 
╶─ No changes have been performed

$ cat packages/test/test.1/opam
opam-version: "2.0"
substs: "does-not-exist"
$ opam reinstall test
The following actions will be performed:
=== recompile 1 package
  ↻ test 1

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>

#=== ERROR while compiling test.1 =============================================#
String expansion failed for these files:
  - does-not-exist.in: OpamSystem.File_not_found("/home/kit_ty_kate/.opam/5.3/.opam-switch/build/test.1/does-not-exist.in")



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build test 1
└─ 
╶─ No changes have been performed

In these examples, the presence of a non-existent file in either substs, patches or extra-files causes opam to display the raw exception instead of a more reasonable error message