Reword "crate not found" resolve message

```
error[E0432]: unresolved import `some_novel_crate`
 --> file.rs:1:5
  |
1 | use some_novel_crate::Type;
  |     ^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `some_novel_crate`
```

On resolve errors where there might be a missing crate, mention `cargo add foo`:

```
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `nope`
  --> $DIR/conflicting-impl-with-err.rs:4:11
   |
LL | impl From<nope::Thing> for Error {
   |           ^^^^ use of unresolved module or unlinked crate `nope`
   |
   = help: if you wanted to use a crate named `nope`, use `cargo add nope` to add it to your `Cargo.toml`
```

1 files changed, 2 insertions, 1 deletions

diff --git a/tests/ui/parser/mod_file_not_exist_windows.rs b/tests/ui/parser/mod_file_not_exist_windows.rs
index 88780c0c24e..bb74684d994 100644
--- a/tests/ui/parser/mod_file_not_exist_windows.rs
+++ b/tests/ui/parser/mod_file_not_exist_windows.rs
@@ -5,5 +5,6 @@ mod not_a_real_file; //~ ERROR file not found for module `not_a_real_file`
 
 fn main() {
     assert_eq!(mod_file_aux::bar(), 10);
-    //~^ ERROR failed to resolve: use of undeclared crate or module `mod_file_aux`
+    //~^ ERROR failed to resolve: use of unresolved module or unlinked crate `mod_file_aux`
+    //~| HELP you might be missing a crate named `mod_file_aux`
 }