bug#30645: Uncaught exception in cuirass

[Andreas Enge <andreas@enge.fr>]

Hello,

an error I am seeing from time to time in my cuirass.log file when
testing bayfront:

Uncaught exception in fiber ##f:
In ice-9/boot-9.scm:
    829:9  4 (catch _ _ #<procedure 7f8567e4c7d0 at cuirass/base.sc?> ?)
In cuirass/base.scm:
   599:26  3 (_)
    531:2  2 (build-packages _ #<<sqlite-db> pointer: #<pointer 0x7?> ?)
   381:27  1 (spawn-builds #<build-daemon 256.97 622e7910> #<<sqlit?> ?)
In srfi/srfi-1.scm:
   414:16  0 (split-at _ _)
srfi/srfi-1.scm:414:16: In procedure split-at:
In procedure cdr: Wrong type argument in position 1 (expecting pair): ()

This may be related to bug #30644, since it always (in three cases) follows
a line such as:
2018-02-28T06:26:31 batch of builds (partially) failed:all build users are currently in use; consider creating additional users and adding them to the `guixbuild' group (status: 1)

But conversely, not all lines "batch of builds..." are followed by this
error, and anyway the order of output may be mixed up between different
threads.

Andreas