Unhandled and unclear exception when (included) ruleset is missing

[BartChris]

Describe the bug
If you have a ruleset (e.g. ruleset_mono.xml) which includes another ruleset (e.g. ruleset_languages.xml) which is not present Kitodo throws an unhelpful stacktrace.

Given this include statement:

<ruleset xmlns="http:https://names.kitodo.org/ruleset/v2"
    xmlns:xsi="http:https://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http:https://names.kitodo.org/ruleset/v2 ruleset.xsd" lang="en">
    <include>ruleset_languages.xml</include>
...

if ruleset_languages.xml does not exist the following stacktrace is shown in the UI:

The same stracktrace is shown if the original ruleset ruleset_mono.xmlis missing, so there is also no useful error in the UI for that scenario.

In the Kitodo logfile a more helpful error message is displayed:

[ERROR] 2024-01-09 18:45:25.349 [http-nio-8080-exec-6] CreateProcessForm - Die angegebene Regelsatzdatei ruleset_mono.xml konnte nicht gefunden werden.
org.kitodo.exceptions.RulesetNotFoundException: Die angegebene Regelsatzdatei ruleset_mono.xml konnte nicht gefunden werden.
	at org.kitodo.production.services.data.RulesetService.openRuleset(RulesetService.java:239) ~[classes/:3.7.0-SNAPSHOT]
	at org.kitodo.production.forms.createprocess.CreateProcessForm.updateRulesetAndDocType(CreateProcessForm.java:130) ~[classes/:3.7.0-SNAPSHOT]
	at org.kitodo.production.forms.createprocess.CreateProcessForm.prepareProcess(CreateProcessForm.java:431) ~[classes/:3.7.0-SNAPSHOT]

But this is also not the reason for the error message since ruleset_mono.xml is present. What is missing is ruleset_languages.xml but the error log does not indicate that.

To Reproduce
Steps to reproduce the behavior:

1. Create a ruleset which includes another ruleset which is not present in the file system.
2. Try to create a process with a process template using that ruleset.

Expected behavior
It should be reported (at best in the UI) that the included ruleset file is not present.

Release
Current Master (3.6.1 as well)