backtrace: guix import egg srfi-1

[jgart]

Hi Guixers,

Just wanted to share this backtrace I got when trying to use the chicken 
importer:

```
 λ guix import egg srfi-1
Backtrace:
In ice-9/boot-9.scm:
  1752:10 10 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           9 (apply-smob/0 #<thunk 7f942f09e2e0>)
In ice-9/boot-9.scm:
    724:2  8 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  7 (_ #(#(#<directory (guile-user) 7f942f0a3c80>)))
In guix/ui.scm:
   2247:7  6 (run-guix . _)
  2210:10  5 (run-guix-command _ . _)
In guix/scripts/import.scm:
   124:11  4 (guix-import . _)
In guix/scripts/import/egg.scm:
    98:26  3 (guix-import-egg . _)
In guix/import/egg.scm:
   193:26  2 (egg->guix-package "srf-1" #f #:file _ #:source _)
   120:29  1 (egg-metadata "srf-1" #:version _ #:file _)
In unknown file:
           0 (string-append "/home/jgart/.cache/guix/checkouts/exkl…" …)

ERROR: In procedure string-append:
In procedure string-append: Wrong type (expecting string): #f
```

Here' another:

```
 λ guix import egg srfi-11
Backtrace:
In ice-9/boot-9.scm:
  1752:10 10 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           9 (apply-smob/0 #<thunk 7fd9d5d922e0>)
In ice-9/boot-9.scm:
    724:2  8 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  7 (_ #(#(#<directory (guile-user) 7fd9d5d97c80>)))
In guix/ui.scm:
   2247:7  6 (run-guix . _)
  2210:10  5 (run-guix-command _ . _)
In guix/scripts/import.scm:
   124:11  4 (guix-import . _)
In guix/scripts/import/egg.scm:
    98:26  3 (guix-import-egg . _)
In guix/import/egg.scm:
   193:26  2 (egg->guix-package "srfi-11" #f #:file _ #:source _)
   120:29  1 (egg-metadata "srfi-11" #:version _ #:file _)
In unknown file:
           0 (string-append "/home/jgart/.cache/guix/checkouts/exkl…" …)

ERROR: In procedure string-append:
In procedure string-append: Wrong type (expecting string): #f

```

Should a condition or exception be added for handling these cases?

all best,

jgart