UploadDocs failure should throw an Exception

Currently, when UploadDocs fails, we just log the error and go on with the test. This can throw off test results when comparing among Solr setups, since both setups may end up with more/less docs in their collections. To fix this, I think UploadDocs should throw an Exception if it receives a non-200 status code, or at least "fatal" status codes such as 404. Then, callers should handle this by ending the test preemptively (we may not have to change the callers' code, not sure!).

See: https://github.com/fullstorydev/solr-bench/blob/f8e2567293ff530f2dbf81da9aa57ba4292e2470/src/main/java/org/apache/solr/benchmarks/indexing/UploadDocs.java#L102

Start in src/main/java/org/apache/solr/benchmarks/indexing/UploadDocs.java around line 102 and inspect how non-200 responses are currently logged. Trace its callers to determine whether an exception already ends the benchmark or requires handling. Done means failed uploads stop the test rather than allowing misleading comparisons between Solr setups.

Written by the indexing model from the issue text.