[BUG] app module generation fails entirely when one java module cannot be found

[line-o]

Describe the bug

On exist-db.org the crypto module was reported as installed but could not be found in the expath repo.
Because of this the (re-)generation of the function documentation failed so that only a few modules and their functions was available for searching and browsing.

The call to https://exist-db.org/exist/apps/fundocs/modules/reindex.xql returned Status 400 and

<?xml version="1.0" ?>
<exception>
    <path>/db/apps/fundocs/modules/reindex.xql</path>
    <message>exerr:ERROR Cannot find module class from EXPath repository: org.expath.exist.crypto.ExistExpathCryptoModule [at line 54, column 53, source: /db/apps/fundocs/modules/scan.xql]
    In function:
        docs:load-internal-modules($store as function(*)) as item()* [77:9:/db/apps/fundocs/modules/scan.xql]
        docs:load-fundocs($target as xs:string) as item()* [13:39:/db/apps/fundocs/modules/scan.xql]</message>
</exception>

Expected behavior

Even if a single module is broken all other modules should generate their documentation.

To Reproduce

This issue is really hard to reproduce because that would mean deliberately breaking the expath repo.
Maybe installing crypto library and then manually removing it from expath repo on disk works.

Context (please always complete the following information):

• OS: N/A
• eXist-db Version: 6.4.0
• Java Version: Java8u312
• App Version: 1.3.1

Additional context

• How is eXist-db installed? N/A
• Any custom changes in e.g. conf.xml? N/A

[line-o]

should be fixed by #105