Check if module has been registered when referenced by a file handler

[wagnert]

Actually there is a chance to misconfigure file handlers by specifying the name of a not existing module, like

<fileHandlers>
    <fileHandler name="<INVALID_MODULE_NAME>" extension=".do" />
</fileHandlers>

This actually would invoke the ProxyModule which triggers an error because of a missing upstream configuration (by default, appserver.xml has NO upstream configuration).

To avoid misconfigurations, the CoreModule should check, if the module, a file handler references is available or not. If not, a 500 error should be triggered and a debug message should be written to the error log.

Additional Optimization:

It would make sense, to NOT hardcode module names, but instead make them configurable in appserver.xml like

<modules>
    <module name="vhost" type="\AppserverIo\WebServer\Modules\VirtualHostModule"/>
    <module name="env" type="\AppserverIo\WebServer\Modules\EnvironmentVariableModule" />
    ...
</modules>

These names can then be used for the file handler configuration later on, without the need to know the modules names from the source code.