Presumably this should throw an HTTP 400 error with some sensible text instead?

The exception is not thrown in the strict sense, it is caught and rendered via wfThumbError( 500, $e->getHTML() ). Should we display a human-readable error message instead of the backtrace? Seems helpful for debugging.

ImageHandler::makeParamString() throws MWException when the width is missing; child classes return false instead, and that is handled nicely (compare https://commons.wikimedia.org/w/thumb.php?f=Example.svg ). So there are two ways to fix this:

1. have ImageHandler::makeParamString() return false - violates the function signature (as documented at MediaHandler::makeParamString()) but child classes do that already so this at least makes it consistent
2. create a new exception subclass

In T88508#1016745, @Tgr wrote:

have ImageHandler::makeParamString() return false

That does not seem to be handled correctly by the methods calling makeParamString, so better not mess with it.

Change 188730 had a related patch set uploaded (by Gergő Tisza):
Handle missing width nicely in thumb.php

https://gerrit.wikimedia.org/r/188730

Patch-For-Review

Change 188730 merged by jenkins-bot:
Handle missing width nicely in thumb.php

https://gerrit.wikimedia.org/r/188730

Fix confirmed: http://en.wikipedia.beta.wmflabs.org/w/thumb.php?f=Bust_orant_Louvre_AO17557_local.jpg&h=100