Fix #510: Propagate IOException instead of swallowing it in ApplicationXmlWriter.write()

[elharo]

Closes #510

Problem

In ApplicationXmlWriter.write(), any IOException during XML writing (e.g., disk full, permission denied) is silently swallowed:

```java
} catch (IOException ex) {
// ignore
}
```

This means the EAR build succeeds with a truncated or missing application.xml deployment descriptor, making the failure undetectable.

Changes

1. ApplicationXmlWriter.java: Replace the empty catch block with a throw of EarPluginException wrapping the IOException.
2. ApplicationXmlWriterTest.java (new): Unit tests for the writer:
   • testWriteSucceedsToValidFile: happy path writing to a temp file
   • testWriteThrowsExceptionOnIoError: writing to /dev/full asserts EarPluginException is thrown

Verification

• Test-first: the new test failed before the fix (exception was silently swallowed)
• After fix: all 45 tests pass