Improve the error message for missing dependencies.

I've changed the error message that appears when a dependency can't be located. With my changes, mismatches between Cargo.toml dependency names and actual dependency package names are (slightly) more apparent. This doesn't change the error that appears when a dependency has an invalid source.

Example:

```toml
# Cargo.toml
...
[dependencies.foo]

path = "foo"
```

```toml
# foo/Cargo.toml
[package]

name = "not_foo"
```

Previously:

```
$ cargo build
No package named Dependency { name: foo, namespace: file:/home/michael/cargo_test, req: *, transitive: true } found
```

Now:

```
$ cargo build
No package named `foo` found (required by `bar`).
Location searched: /some/folder/not_foo
Version required: *
```

In verbose mode I think it would be also nice to print a list of packages that Cargo *does* know about. Something like: "Known packages amongst dependencies: not_foo, etc"