Improve error message in GenomicsDBImport (#7375)

* The UserException when failing to open a FeatureReader now includes the message from the underlying error.
It was previously hard to understand WHY we had failed to open a reader.  See #7362 for an example

src/main/java/org/broadinstitute/hellbender/tools/genomicsdb/GenomicsDBImport.java

@@ -1010,7 +1010,8 @@ public VariantContext next() {
                 }
             };
         } catch (final TribbleException e){
-            throw new UserException("Failed to create reader from " + variantURI, e);
+            throw new UserException("Failed to create reader from " + variantURI + " because of the following error:\n\t"
+                    + e.getMessage(), e);
         }
     }