Change error message for devbox add when a package is not found (#1088)

## Summary
Change error message for devbox add when a package is not found. To
repro:
`devbox add hellow@latest`

Original output:
```
Error: No results found for "hellow@latest".
```

New output:
```
Error: No results found for "hellow@latest".

To search for packages, use the `devbox search` command
```

## How was it tested?
```
devbox run build
devbox add hellow@latest
```

Author: LucilleH

internal/boxcli/add.go

@@ -13,7 +13,7 @@ import (
 	"go.jetpack.io/devbox/internal/nix"
 )
 
-const toSearchForPackages = "To search for packages use https://search.nixos.org/packages"
+const toSearchForPackages = "To search for packages, use the `devbox search` command"
 
 type addCmdFlags struct {
 	config configFlags

internal/searcher/client.go

@@ -14,6 +14,7 @@ import (
 	"go.jetpack.io/devbox/internal/boxcli/usererr"
 	"go.jetpack.io/devbox/internal/envir"
 	"go.jetpack.io/devbox/internal/lock"
+	"go.jetpack.io/devbox/internal/nix"
 )
 
 const searchAPIEndpoint = "https://search.devbox.sh"
@@ -60,7 +61,7 @@ func (c *client) Resolve(pkg string) (*lock.Package, error) {
 		return nil, err
 	}
 	if len(result.Results) == 0 {
-		return nil, usererr.New("No results found for %q.", pkg)
+		return nil, nix.ErrPackageNotFound
 	}
 	return &lock.Package{
 		LastModified: result.Results[0].Packages[0].Date,