Improve loading/indexing feedback

[leonardossz]

The loading process can take quite a long time hence the user needs some kind of feedback if a query is issued while loader is still running.

[leonardossz]

Also we need to store previous loader failures to let API users know that stargraph did not fulfill the intended request. Here is a clear case where this is happening though the user still receives a successful acknowledge. Remember that loader is asynchronous by design.

2017-07-20 18:10:21,610 INFO  [index] Loading dbpedia-2016.documents, [reset=true, limit=-1] 
2017-07-20 18:10:21,610 WARN  [index] Old data will be DELETED! 
2017-07-20 18:10:21,627 ERROR [core] Failed to load documents from file /home/lsouza/StarGraph/data/dbpedia-2016/documents/documents.json. 
2017-07-20 18:10:21,627 ERROR [index] Loader failure. 
net.stargraph.StarGraphException: Fail to initialize data provider: dbpedia-2016.documents
	at net.stargraph.core.Stargraph.createDataProvider(Stargraph.java:239)
	at net.stargraph.core.index.BaseIndexer.doBeforeLoad(BaseIndexer.java:177)
	at net.stargraph.core.index.BaseIndexer.lambda$doLoad$0(BaseIndexer.java:239)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:748)
Caused by: net.stargraph.StarGraphException: java.io.FileNotFoundException: File '/home/lsouza/StarGraph/data/dbpedia-2016/documents/documents.json' does not exist
	at net.stargraph.core.DocumentIterator.<init>(DocumentIterator.java:69)
	at net.stargraph.core.DocumentProviderFactory.create(DocumentProviderFactory.java:45)
	at net.stargraph.core.Stargraph.createDataProvider(Stargraph.java:230)
	... 7 common frames omitted
Caused by: java.io.FileNotFoundException: File '/home/lsouza/StarGraph/data/dbpedia-2016/documents/documents.json' does not exist
	at org.apache.commons.io.FileUtils.openInputStream(FileUtils.java:299)
	at org.apache.commons.io.FileUtils.lineIterator(FileUtils.java:1856)
	at net.stargraph.core.DocumentIterator.<init>(DocumentIterator.java:65)
	... 9 common frames omitted
2017-07-20 18:10:21,628 INFO  [index] Loader is finishing.. 
2017-07-20 18:10:21,628 INFO  [index] Waiting for transport to serialize all remaining documents. 
2017-07-20 18:10:21,628 INFO  [index] Optimizing index for reading.. 
2017-07-20 18:10:21,634 INFO  [index] Loader is done.