Make TestModule.Output print a list of all outputs on error

Debugging when writing tests using TestModule.Outputs is
unnecessarily hard, it panics when an incorrect output path
is given but doesn't provide any help to figure out why.
Follow the pattern used by TestContext.ModuleForTests
and print the list of valid output paths on failure.

Test: m checkbuild
Change-Id: I50e8e2dfc2070bd538d47cf6495a489f727b1564