Fix errors.TestFormatGeneric

[vtopc]

Fix test/code or deprecate and switch to built-in package errors.

=== RUN   TestFormatGeneric
    format_test.go:476: test 1: block 6: fmt.Sprintf("%+v", err):
        got:
        "github.com/mailgun/holster/v4/errors.testGenericRecursive\n\tgithub.com/mailgun/holster/v4/errors/format_test.go:504\ngithub.com/mailgun/holster/v4/errors.testGenericRecursive\n\tgithub.com/mailgun/holster/v4/errors/format_test.go:523\ngithub.com/mailgun/holster/v4/errors.testGenericRecursive\n\tgithub.com/mailgun/holster/v4/errors/format_test.go:523\ngithub.com/mailgun/holster/v4/errors.testGenericRecursive\n\tgithub.com/mailgun/holster/v4/errors/format_test.go:523\ngithub.com/mailgun/holster/v4/errors.TestFormatGeneric\n\tgithub.com/mailgun/holster/v4/errors/format_test.go:359\ntesting.tRunner\n\ttesting/testing.go:1446\nruntime.goexit\n\truntime/asm_amd64.s:1594"
        want:
        "github.com/mailgun/holster/v4/errors.(func·002|TestFormatGeneric.func2)\n\t.+/errors/format_test.go:333"
        all-got:
           new-error
           github.com/mailgun/holster/v4/errors.TestFormatGeneric
        	github.com/mailgun/holster/v4/errors/format_test.go:315
        testing.tRunner
        	testing/testing.go:1446
        runtime.goexit
        	runtime/asm_amd64.s:1594
           with-message
           with-message
           with-message
           github.com/mailgun/holster/v4/errors.testGenericRecursive
        	github.com/mailgun/holster/v4/errors/format_test.go:504
        github.com/mailgun/holster/v4/errors.testGenericRecursive
        	github.com/mailgun/holster/v4/errors/format_test.go:523
        github.com/mailgun/holster/v4/errors.testGenericRecursive
        	github.com/mailgun/holster/v4/errors/format_test.go:523
        github.com/mailgun/holster/v4/errors.testGenericRecursive
        	github.com/mailgun/holster/v4/errors/format_test.go:523
        github.com/mailgun/holster/v4/errors.TestFormatGeneric
        	github.com/mailgun/holster/v4/errors/format_test.go:359
        testing.tRunner
        	testing/testing.go:1446
        runtime.goexit
        	runtime/asm_amd64.s:1594
        all-want:
           new-error
           github.com/mailgun/holster/v4/errors.TestFormatGeneric
        	.+/errors/format_test.go:315
           with-message
           with-message
           with-message
           github.com/mailgun/holster/v4/errors.(func·002|TestFormatGeneric.func2)
        	.+/errors/format_test.go:333
--- FAIL: TestFormatGeneric (0.00s)

https://github.com/mailgun/holster/actions/runs/3740540806/jobs/6349033540