config and command: use errwrap to propagate config load errors

Previously we were using fmt.Sprintf and thus forcing the stringification
of the wrapped error.

Using errwrap allows us to unpack the original error at the top of the
stack, which is useful when the wrapped error is really a hcl.Diagnostics
containing potentially-multiple errors and possibly warnings.

command/meta_new.go

@@ -47,7 +47,7 @@ func (m *Meta) Module(path string) (*module.Tree, error) {
 
 	err = mod.Load(m.moduleStorage(m.DataDir()), module.GetModeNone)
 	if err != nil {
-		return nil, fmt.Errorf("Error loading modules: %s", err)
+		return nil, errwrap.Wrapf("Error loading modules: {{err}}", err)
 	}
 
 	return mod, nil

config/import_tree.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"io"
 	"os"
+
+	"github.com/hashicorp/errwrap"
 )
 
 // configurable is an interface that must be implemented by any configuration
@@ -125,10 +127,7 @@ func (t *importTree) Close() error {
 func (t *importTree) ConfigTree() (*configTree, error) {
 	config, err := t.Raw.Config()
 	if err != nil {
-		return nil, fmt.Errorf(
+		return nil, errwrap.Wrapf(fmt.Sprintf("Error loading %s: {{err}}", t.Path), err)
-			"Error loading %s: %s",
-			t.Path,
-			err)
 	}
 
 	// Build our result