[Core] Handle audio exceptions more gracefully

[cweitkamp]

• Restore volume only if it was set before
• Handle exceptions more gracefully
• Minor improvements in documentation

An IOException from getVolume() or setVolume() methods leads to a stack trace log. The warning itself should be sufficient for the user.

2018-03-18 10:00:44.326 [WARN ] [core.audio.internal.AudioManagerImpl] - An exception occurred while setting the volume of sink webaudio : 'Web Audio sink does not support volume level changes.'
java.io.IOException: Web Audio sink does not support volume level changes.
	at org.eclipse.smarthome.io.webaudio.internal.SSEAudioSink.setVolume(SSEAudioSink.java:115) ~[?:?]
	at org.eclipse.smarthome.core.audio.internal.AudioManagerImpl.setVolume(AudioManagerImpl.java:180) ~[?:?]
	at org.eclipse.smarthome.core.audio.internal.AudioManagerImpl.play(AudioManagerImpl.java:115) ~[?:?]
	at org.eclipse.smarthome.core.audio.internal.AudioManagerImpl.playFile(AudioManagerImpl.java:144) ~[?:?]
	at org.eclipse.smarthome.core.audio.internal.AudioManagerImpl.playFile(AudioManagerImpl.java:124) ~[?:?]
	at org.eclipse.smarthome.model.script.actions.Audio.playSound(Audio.java:40) ~[?:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:?]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.invokeOperation(XbaseInterpreter.java:1085) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.invokeOperation(XbaseInterpreter.java:1060) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter._invokeFeature(XbaseInterpreter.java:1046) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.invokeFeature(XbaseInterpreter.java:991) ~[?:?]
	at org.eclipse.smarthome.model.script.interpreter.ScriptInterpreter.invokeFeature(ScriptInterpreter.java:143) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter._doEvaluate(XbaseInterpreter.java:901) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter._doEvaluate(XbaseInterpreter.java:864) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.doEvaluate(XbaseInterpreter.java:223) ~[?:?]
	at org.eclipse.smarthome.model.script.interpreter.ScriptInterpreter.doEvaluate(ScriptInterpreter.java:219) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.internalEvaluate(XbaseInterpreter.java:203) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter._doEvaluate(XbaseInterpreter.java:446) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.doEvaluate(XbaseInterpreter.java:227) ~[?:?]
	at org.eclipse.smarthome.model.script.interpreter.ScriptInterpreter.doEvaluate(ScriptInterpreter.java:219) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.internalEvaluate(XbaseInterpreter.java:203) ~[?:?]
	at org.eclipse.xtext.xbase.interpreter.impl.XbaseInterpreter.evaluate(XbaseInterpreter.java:189) ~[?:?]
	at org.eclipse.smarthome.model.script.runtime.internal.engine.ScriptImpl.execute(ScriptImpl.java:82) ~[?:?]
	at org.eclipse.smarthome.model.rule.runtime.internal.engine.RuleEngineImpl.lambda$2(RuleEngineImpl.java:339) ~[?:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:?]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) [?:?]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) [?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:?]
	at java.lang.Thread.run(Thread.java:748) [?:?]

See https://community.openhab.org/t/web-audio-sink-does-not-support-volume-level-changes/41989

Signed-off-by: Christoph Weitkamp github@christophweitkamp.de