FetcherException supports outputting http information on failure: org.jabref.logic.importer.FetcherException#getLocalizedMessage

This should be used at URLDownload (e.g., org.jabref.logic.net.URLDownload#asString(java.nio.charset.Charset, java.net.URLConnection))

Task:

1. Adapt org.jabref.logic.importer.fetcher.DoiFetcher#performSearchById to use org.jabref.logic.importer.fileformat.BibtexParser#parseEntries instead of singleFromString
2. Make asString(Charset encoding, URLConnection connection) private
3. Adapt all other methods

Note: You can merge asString(Charset encoding, URLConnection connection) with asString(URLConnection connection) probably.