Bubble Up regexp Error For BodyMatchString

[midnightconman]

This should make fixing regex pattern issues easier... to bubble up an error like this:

https://play.golang.org/p/dubZiQ-HCN0

[h2non]

Tests are failing. Can you take a look?

[midnightconman]

Sorry this took so long, I missed this PR 😺

I am not sure this test failure is related to my changes, what do you think?

--- FAIL: TestJSONSchemaInvalid (0.00s)
	<autogenerated>:1: 
		json_schema_test.go:67: should be == 
		 	have: (bool) false
			want: (bool) true

I don't see any error handling in the code base around the function I changed... can you take a look and let me know if you can find any additional info?