Promoted new cleaner error into the dune pkg test

Signed-off-by: Ambre Austen Suhamy <ambre@tarides.com>

test/blackbox-tests/test-cases/pkg/invalid-version.t

@@ -10,5 +10,12 @@ it in a dune-project file.
   >  (depends foo.1.2.3))
   > EOF
 
-  $ dune pkg lock 2>&1 | head -n1
-  Error: exception Failure("Invalid character in package name \"foo.1.2.3\"")
+  $ dune pkg lock 2>&1
+  File "dune-project", line 4, characters 10-19:
+  4 |  (depends foo.1.2.3))
+                ^^^^^^^^^
+  Error: "foo.1.2.3" is an invalid opam package name.
+  Package names can contain letters, numbers, '-', '_' and '+', and need to
+  contain at least a letter.
+  Hint: foo_1_2_3 would be a correct opam package name
+  [1]