build: include position information in errors

Updates #5

build/error.go

@@ -0,0 +1,30 @@
+package build
+
+import (
+	"github.com/mmcloughlin/avo/internal/stack"
+	"github.com/mmcloughlin/avo/src"
+)
+
+// Error represents an error during building, optionally tagged with the position at which it happened.
+type Error struct {
+	Position src.Position
+	Err      error
+}
+
+// exterr constructs an Error with position derived from the first frame in the
+// call stack outside this package.
+func exterr(err error) Error {
+	e := Error{Err: err}
+	if f := stack.ExternalCaller(); f != nil {
+		e.Position = src.FramePosition(*f).Relwd()
+	}
+	return e
+}
+
+func (e Error) Error() string {
+	msg := e.Err.Error()
+	if e.Position.IsValid() {
+		return e.Position.String() + ": " + msg
+	}
+	return msg
+}