Descriptive errror when `some/specifier` does not match entry for `some`

For example:

```
error: Relative import path "@std/http/http-errors" not prefixed with / or ./ or ../ and not in import map from "file:///home/runner/work/deno-slack-api/deno-slack-api/deno-slack-api/src/base-client.ts" at file:///home/runner/work/deno-slack-api/deno-slack-api/deno-slack-api/src/base-client.ts:7:49
```

...when the import map only has `@std/http`. (and not `@std/http/`)

This is not descriptive enough. It should provide actionable feedback.