Updated build file parser error message when the same target is redefined

In our repo we make use of the BUILD.<suffix> style BUILD files and sometimes there is an error where someone has 2 files with the same target.  Here is the error message users get:

AssertionError: BuildFileAddress(geocoding/BUILD.gen, geocoding) already in BuildGraph._target_proxy_by_address even though this BUILD file has not yet been added to the BuildGraph.  The target type is: <class 'pants.targets.jvm_binary.JvmBinary'>

I have attempted to improve the usability to end users:

pants.base.build_file_parser.SiblingConflictException: Both geocoding/BUILD and geocoding/BUILD.gen define the same target 'geocoding'

I also created a separate exception for when the same target is defined twice in the same file.