RANGER-5338: Make XMLUtils's error message more informative

[okumin]

What changes were proposed in this pull request?

This PR would add more contexts to error logs when failing to load a configuration file, such as ranger-admin-site.xml.

https://issues.apache.org/jira/browse/RANGER-5338

How was this patch tested?

I executed this test case and confirmed that the following error message showed up.

ERROR org.apache.ranger.plugin.util.XMLUtils -- Error loading : non-existent-file.xml
java.io.FileNotFoundException: non-existent-file.xml is not found
	at org.apache.ranger.plugin.util.XMLUtils.getFileInputStream(XMLUtils.java:139)
	at org.apache.ranger.plugin.util.XMLUtils.loadConfig(XMLUtils.java:51)
	at org.apache.ranger.common.TestRangerProperties.testXMLUtilsLoadConfigNoFileGraceful(TestRangerProperties.java:61)