internal/test: switch errors to error classes

[jtolio]

if you construct an error directly at package init time, you
won't get useful stack traces or anything. zeebo/errs expects that
you always construct an error (.New) when the error actually
happens. instead, you call Class at init time, then use (Class).Has
to test for error type membership.

---

This change is 

[coyle]

Reviewed 4 of 4 files at r1.
Review status: complete! all files reviewed, all discussions resolved (waiting on @bryanchriswhite)

---

Comments from Reviewable

[coyle]

---

Review status: complete! all files reviewed, all discussions resolved (waiting on @bryanchriswhite)

---

Comments from Reviewable

[kaloyan-raev]

Review status: all files reviewed, 1 unresolved discussion (waiting on @jtolds and @bryanchriswhite)

---

storage/redis/client.go, line 15 at r1 (raw file):

var (
	Error = errs.Class("redis error")

The linter complains of missing comment and fails the build.

---

Comments from Reviewable

[jtolio]

Review status: all files reviewed, 1 unresolved discussion (waiting on @jtolds and @bryanchriswhite)

---

storage/redis/client.go, line 15 at r1 (raw file):

Previously, kaloyan-raev (Kaloyan Raev) wrote…

The linter complains of missing comment and fails the build.

doh

---

Comments from Reviewable

[bryanchriswhite]

Reviewed 3 of 4 files at r1, 1 of 1 files at r3.
Review status: all files reviewed, 1 unresolved discussion (waiting on @kaloyan-raev)

---

Comments from Reviewable

[bryanchriswhite]

---

Review status: all files reviewed, 1 unresolved discussion (waiting on @kaloyan-raev)

---

Comments from Reviewable

[kaloyan-raev]

Reviewed 3 of 4 files at r1, 1 of 1 files at r3.
Review status: complete! all files reviewed, all discussions resolved

---

Comments from Reviewable

[kaloyan-raev]

---

Review status: complete! all files reviewed, all discussions resolved

---

Comments from Reviewable