Prior to the shellbox migration, if during the parsing of a page,
pygmentize failed (i.e. non-zero exit from its local shell command,
pretty much the only way a php shell exec could fail), then
SyntaxHighlight would fallback to outputting a preformatted plain
`<pre>`.
The logic still exists in the code, and is still triggered for cases
where the command reached shellbox and its result was "successfully"
communicated to MediaWiki (HTTP 200), with the boxed result reporting
the non-zero exit code on the shellbox server.
However, the more likely scenario in the new setup is that the command
times out or never reaches the server in the first place, in which
case we don't get any shell exit code. Instead, we get a Shellbox
exception since the result is unknowable.
Instead of fatalling the entire pageview with a PHP exception and
HTTP 500 from MW, use the same graceful fallback.
Bug: T292663
Change-Id: Icaa8c34ff97ad8a99d044beab529ef943071269c
