build: add package ID to compiler and optimization error messages

This improves error reporting slightly.

builder/error.go

@@ -3,7 +3,8 @@ package builder
 // MultiError is a list of multiple errors (actually: diagnostics) returned
 // during LLVM IR generation.
 type MultiError struct {
-	Errs []error
+	ImportPath string
+	Errs       []error
 }
 
 func (e *MultiError) Error() string {
@@ -15,14 +16,15 @@ func (e *MultiError) Error() string {
 // newMultiError returns a *MultiError if there is more than one error, or
 // returns that error directly when there is only one. Passing an empty slice
 // will lead to a panic.
-func newMultiError(errs []error) error {
+// The importPath may be passed if this error is for a single package.
+func newMultiError(errs []error, importPath string) error {
 	switch len(errs) {
 	case 0:
 		panic("attempted to create empty MultiError")
 	case 1:
 		return errs[0]
 	default:
-		return &MultiError{errs}
+		return &MultiError{importPath, errs}
 	}
 }