Manifest generation: fail on write errors, clean up logging

[rossbacher]

Summary

• ManifestGenerator now throws DeepLinkProcessorException on IOException when writing the generated AndroidManifest.xml, so write failures fail the build cleanly instead of being swallowed as a diagnostic warning that surfaces later in a confusing place. Catch is scoped to IOException so genuine processor bugs (NPE/ISE) are no longer hidden.
• RelocateDeepLinkManifestTask: use orNull on the @optional kspManifestFile property (get() NPEs when unset), replace println with logger.warn so the message respects Gradle log level, collapse an empty else-if branch, and fix a couple of typos.
• GenerateManifestIntentFiltersForDeeplinkDispatchTask: replace println with logger.warn, and replace error(...) (IllegalStateException) with throw GradleException(...) for idiomatic build-failure output.

Test plan

• ./gradlew :deeplinkdispatch-processor:test :deeplinkdispatch-gradle-plugin:test
• ./gradlew :sample:testDebugUnitTest :deeplinkdispatch:test :deeplinkdispatch-base:test