Share-Based Payments - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Oct. 31, 2017	Apr. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	2,308,704
Shares, Granted	194,977
Shares, Exercised	0
Shares, Forfeited	(6,042)
Shares, Canceled	0
Shares, Expired	(2,709)
Shares, Outstanding, Ending Balance	2,494,930	2,308,704
Shares, Vested and expected to vest	2,494,930
Shares, Exercisable	2,425,225
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted Average Exercise Price, Outstanding, Beginning Balance (in usd per share)	$ 2.86
Weighted Average Exercise Price, Granted (in usd per share)	2.51
Weighted Average Exercise Price, Exercised (in usd per share)	0
Weighted Average Exercise Price, Forfeited (in usd per share)	7.58
Weighted Average Exercise Price, Canceled (in usd per share)	0.00
Weighted Average Exercise Price, Expired (in usd per share)	8.44
Weighted Average Exercise Price, Outstanding, Ending Balance (in usd per share)	2.82	$ 2.86
Weighted Average Exercise Price, Vested and expected to vest (in usd per share)	2.82
Weighted Average Exercise Price, Exercisable (in usd per share)	$ 2.81
Weighted Average Remaining Contractual Term, Outstanding, Beginning Balance (in years)		6 years 1 month 6 days
Weighted Average Remaining Contractual Life, Granted (in years)	9 years 8 months 12 days
Weighted Average Remaining Contractual Term, Outstanding, Ending Balance (in years)	5 years 10 months 25 days
Weighted Average Remaining Contractual Life, Vested and expected to vest (in years)	5 years 10 months 25 days
Weighted Average Remaining Contractual Life, Exercisable (in years)	5 years 10 months 25 days
Aggregate Intrinsic Value, Outstanding, Beginning Balance	$ 1,282
Aggregate Intrinsic Value, Granted
Aggregate Intrinsic Value, Outstanding, Ending Balance	2,512	$ 1,282
Aggregate Intrinsic Value, Vested and expected to vest	2,512
Aggregate Intrinsic Value, Exercisable	$ 2,484
Non-employees
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	50,000
Shares, Granted	0
Shares, Exercised	0
Shares, Forfeited	0
Shares, Canceled	0
Shares, Expired	0
Shares, Outstanding, Ending Balance	50,000	50,000
Shares, Vested and expected to vest	50,000
Shares, Exercisable	33,336
Directors and employees
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Outstanding, Beginning Balance	2,258,704
Shares, Granted	194,977
Shares, Exercised	0
Shares, Forfeited	(6,042)
Shares, Canceled	0
Shares, Expired	(2,709)
Shares, Outstanding, Ending Balance	2,444,930	2,258,704
Shares, Vested and expected to vest	2,444,930
Shares, Exercisable	2,391,889