[MASSEMBLY-477] Add option "failOnMissingClassifierArtifact" for multimodule projects

**[Petr Novak](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kek)** opened **[MASSEMBLY-477](https://issues.apache.org/jira/browse/MASSEMBLY-477?redirect=false)** and commented

I have a large multimodule project that needs to create compact distribution files at the root of the modules hierarchy. I want to use the assembly plugin with predefined descriptors for merging submodules classes, sources, tests, resources, configuration, etc. The following descriptor shows the sample of my definition. But there is an issue with usage of \, when used and some submodule does not support this classifier, than the

```java
InvalidAssemblerConfigurationException( "Cannot find attachment with classifier: " + classifier + " in module project: " + project.getId() + ". Please exclude this module from the module-set." )
```

is thrown. It will be great, that the new \ could be supported - it is inspired by the sameoption in maven-dependency-plugin: http://maven.apache.org/plugins/maven-dependency-plugin/unpack-dependencies-mojo.html#failOnMissingClassifierArtifact

```xml

*-api


sources
false
/
true
false

```

At this time I patched the org.apache.maven.plugin.assembly.archive.phase.ModuleSetAssemblyPhase.java to ignore exception and provide only logger.info message.

---

**Affects:** 2.2-beta-5

**Attachments:**
- [ModuleSetAssemblyPhase.java](https://issues.apache.org/jira/secure/attachment/12714148/ModuleSetAssemblyPhase.java) (_20.59 kB_)

Start by reading org.apache.maven.plugin.assembly.archive.phase.ModuleSetAssemblyPhase.java and the attached patched version, then compare the requested option with Maven Dependency Plugin's failOnMissingClassifierArtifact behavior. Done means the assembly descriptor accepts the option and a missing classifier artifact is handled according to its setting instead of always failing.

Written by the indexing model from the issue text.