Improve error message if nested array is incorrect

[linusthe3rd]

If a user messes up their nested array in collectPuts, the error message provided is not that helpful as it's written to explain incorrect use of types, but not why a specific assertion failed:

redux-saga-test-engine/src/core.js

Lines 115 to 119 in 6b4e649

	assert(
	isMap(envMapping) || isNestedArray(envMapping),
	'The second parameter must be a nested array, ' +
	'Map or object containing the same under `mapping` key'
	)

It would be better if a secondary assertion for the nested array was added to explicitly explain why the array approach failed with an error message of something like:

"The nested arrays must have exactly two arguments".

[timbuckley]

@linusthe3rd This is a good suggestion! I'd be happy to have a PR to that effect if you are so inclined 😃