More informative parser error messages

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/146)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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See this question about parser error messages not being very helpful:

http://answers.gazebosim.org/question/15485/tools-to-validate-sdf-files-with/

The error is coming from here:

https://github.com/osrf/sdformat/blob/14daa32dcd3cb86f4b4648d438213479b017dc37/src/parser.cc#L305

Looking at the TinyXml documentation, it looks like there are more functions which we could use to provide a better indication of the error:

http://www.grinninglizard.com/tinyxmldocs/classTiXmlDocument.html

Start at src/parser.cc around line 305, where the current parser error is generated, then consult the linked TinyXmlDocument documentation for additional error details. The issue is complete when parser failures provide a more useful indication of what went wrong than the existing message.

Written by the indexing model from the issue text.