installer produces unhelpful error message when /boot/efi is mounted read-only

In addition to the unhelpful error message, spawning a graphical error message in response to a command-line apt-get dist-upgrade is also unhelpful.

ProblemType: Package
DistroRelease: Ubuntu 15.10
Package: shim-signed 1.15~15.10.1+0.8-0ubuntu2
ProcVersionSignature: Ubuntu 4.2.0-39.46-generic 4.2.8-ckt11
Uname: Linux 4.2.0-39-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
BootEFIContents:
 grub.cfg
 grub.cfg,v
 grubx64.efi
 MokManager.efi
 shimx64.efi
Date: Sat Jun 25 17:20:20 2016
DuplicateSignature: package:shim-signed:1.15~15.10.1+0.8-0ubuntu2:subprocess installed post-installation script returned error exit status 1
ErrorMessage: subprocess installed post-installation script returned error exit status 1
InstallationDate: Installed on 2016-03-10 (107 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
RelatedPackageVersions:
 dpkg 1.18.2ubuntu5.1
 apt 1.0.10.2ubuntu3
SourcePackage: shim-signed
Title: package shim-signed 1.15~15.10.1+0.8-0ubuntu2 failed to install/upgrade: subprocess installed post-installation script returned error exit status 1
UpgradeStatus: Upgraded to wily on 2016-04-25 (61 days ago)