Python installation fails if grpc_tools is not installed

When installing from master branch, I'm getting an exception below. Looks like the ImportError exception handling throws an exception itself. I'll manually install grpc_tools and try again but the handling of ImportError has issues.
 
```
Traceback (most recent call last):
      File "/root/apache-beam-custom/packages/beam/sdks/python/gen_protos.py", line 292, in generate_proto_files
        from grpc_tools import protoc
    ModuleNotFoundError: No module named 'grpc_tools'
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "/opt/conda/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
        self.run()
      File "/opt/conda/lib/python3.7/multiprocessing/process.py", line 99, in run
        self._target(*self._args, **self._kwargs)
      File "/root/apache-beam-custom/packages/beam/sdks/python/gen_protos.py", line 378, in _install_grpcio_tools_and_generate_proto_files
        generate_proto_files(force=force)
      File "/root/apache-beam-custom/packages/beam/sdks/python/gen_protos.py", line 315, in generate_proto_files
        protoc_gen_mypy = _find_protoc_gen_mypy()
      File "/root/apache-beam-custom/packages/beam/sdks/python/gen_protos.py", line 233, in _find_protoc_gen_mypy
        (fname, ', '.join(search_paths)))
    RuntimeError: Could not find protoc-gen-mypy in /root/apache-beam-custom/bin, /root/apache-beam-custom/bin, /usr/local/bin, /opt/conda/bin, /usr/local/sbin, /usr/local/bin, /usr/sbin, /usr/bin, /sbin, /bin
```

Imported from Jira [BEAM-9508](https://issues.apache.org/jira/browse/BEAM-9508). Original Jira may contain additional context.
Reported by: davidyan.