read of /proc/uptime could fail and cause exit

As i noticed when reviewing changes for bug 1708192, I noticed that
the read of /proc/uptime could fail.

This is re-creatable with something like this:
$ mp=/tmp/my.d
$ mkdir -p $mp
$ sudo mount --bind / $mp

# need /dev/ mounted for 'logger' usage (/dev/log) which is also fatal
$ sudo mount --bind /dev $mp/dev

# need /run because in systemd systems, /dev/log is a link to
# /run/systemd/journal/dev-log
$ sudo mount --bind /run $mp/run
$ sudo chroot $mp pollinate --reseed
<13>Aug 3 14:06:32 pollinate[6449]: system was previously seeded at [2017-08-02 18:11:17.357060419 +0000]
<13>Aug 3 14:06:32 pollinate[6449]: client sent challenge to [https://entropy.ubuntu.com/]
grep: /proc/cpuinfo: No such file or directory
/usr/bin/pollinate: 102: /usr/bin/pollinate: cannot open /proc/uptime: No such file
$ echo $?
2