Persist Python interpreter for repeated use

[sevein]

This PR introduces changes to ensure that the Python interpreter is persisted for repeated use, instead of creating a new instance with each method call. This is a low priority change, as it is only beneficial when using *bagit.BagIt repeatedly, e.g.: when validating hundreds of bags.

ErrBusy is returned if you try to use *bagit.BagIt concurrently. If you need to perform concurrent validation you should have a dedicated *bagit.BagIt instance per goroutine. Adding support for concurrent work in *bagit.BagIt would be possible by implementing an internal pool of interpreters, but this does not seem necessary atm.

Using -benchtime=10s shows that the Validate method is significantly faster; within 10 seconds, we can now run Validate approximately 1,200 times compared to the previous 144 times.

Before:

(main) $ go test -run=^BenchmarkValidate -bench=. -benchtime=10s -cpu=1 -count=3
goos: linux
goarch: amd64
pkg: github.com/artefactual-labs/bagit-gython
cpu: Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz
BenchmarkValidate 	     144	  76810063 ns/op
BenchmarkValidate 	     141	  77826339 ns/op
BenchmarkValidate 	     144	  75244650 ns/op
PASS
ok  	github.com/artefactual-labs/bagit-gython	65.955s

After:

(dev/persistent-branch) $ go test -run=^BenchmarkValidate -bench=. -benchtime=10s -cpu=1 -count=3
goos: linux
goarch: amd64
pkg: github.com/artefactual-labs/bagit-gython
cpu: Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz
BenchmarkValidate 	    1171	   9401249 ns/op
BenchmarkValidate 	    1258	   9371621 ns/op
BenchmarkValidate 	    1255	   9158033 ns/op
PASS
ok  	github.com/artefactual-labs/bagit-gython	84.983s

I've made changes to the Python script to improve error handling and reporting. See the following interaction in a terminal where each request (e.g. INVALID-JSON) always receives a JSON-encoded request even when malformed, and errors are now including a type attribute with the name of the underlying Python exception.

$ python main.py
INVALID-JSON
{"err": "Expecting value: line 1 column 1 (char 0)", "type": "JSONDecodeError"}
{}
{"err": "'None' is not a valid command, use: validate, make, exit", "type": "UnknownCommandError"}
{"name": "validate", "args": {"path": "/etc/motd"}}
{"err": "Expected bagit.txt does not exist: /etc/motd/bagit.txt", "type": "BagError"}
{"name": "exit"}

[djjuhasz]

LGTM 👍