cmd/cue/cmd: add test for cue mod get with non-existent module

The error message for this is misleading, as the actual
error is that there are no versions available for the module,
but the error message talks about no major version.

This will be fixed in a subsequent CL.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I1e7a454120beb1b22e52c3175080cb80973cc874
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201492
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/modtidy_not_found.txtar

@@ -0,0 +1,20 @@
+! exec cue mod get other.test@v0
+cmp stderr want-stderr
+
+-- cue.mod/module.cue --
+module: "main.org@v0"
+language: version: "v0.10.0"
+
+-- want-stderr --
+path "other.test" has no major version
+-- main.cue --
+package main
+
+import "other.test/foo"
+x: foo
+
+-- _registry/example.com_v0.0.1/cue.mod/module.cue --
+module: "example.com@v0"
+language: version: "v0.8.0"
+-- _registry/example.com_v0.0.1/top.cue --
+package main