Better error message for certain classes of unresolved imports
Tweak error message

Recommendation of @kud1ing
compile-fail test for new unresolved import error
fix formating and tests
auto merge of #6347 : cmr/rust/unknown_module_resolve_error, r=catamorphism

This improves error reporting for the following class of imports:

```rust
use foo::bar;
```

Where foo, the topmost module, is unresolved. It now results in:

```text
/tmp/foo.rs:1:4: 1:7 error: unresolved import.  perhapsyou forgot an 'extern mod foo'?
/tmp/foo.rs:1 use foo::bar;
                  ^~~
/tmp/foo.rs:1:4: 1:12 error: failed to resolve import: foo::bar
/tmp/foo.rs:1 use foo::bar;
                  ^~~~~~~~
error: failed to resolve imports
error: aborting due to 3 previous errors
```

This is the first of a series of changes I plan on making to unresolved name error messages.