build: return ErrorList type

Adds a very similar interface to go/scanner package for returning a list
of errors.

Updates #34

build/error.go

@@ -1,6 +1,9 @@
 package build
 
 import (
+	"fmt"
+	"log"
+
 	"github.com/mmcloughlin/avo/internal/stack"
 	"github.com/mmcloughlin/avo/src"
 )
@@ -28,3 +31,53 @@ func (e Error) Error() string {
 	}
 	return msg
 }
+
+// ErrorList is a collection of errors for a source file.
+type ErrorList []Error
+
+// Add appends an error to the list.
+func (e *ErrorList) Add(err Error) {
+	*e = append(*e, err)
+}
+
+// addext appends an error to the list, tagged with the
+func (e *ErrorList) addext(err error) {
+	e.Add(exterr(err))
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (e ErrorList) Err() error {
+	if len(e) == 0 {
+		return nil
+	}
+	return e
+}
+
+// An ErrorList implements the error interface.
+func (e ErrorList) Error() string {
+	switch len(e) {
+	case 0:
+		return "no errors"
+	case 1:
+		return e[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", e[0], len(e)-1)
+}
+
+// LogError logs a list of errors, one error per line, if the err parameter is
+// an ErrorList. Otherwise it just logs the err string. Reports at most max
+// errors, or unlimited if max is 0.
+func LogError(l *log.Logger, err error, max int) {
+	if list, ok := err.(ErrorList); ok {
+		for i, e := range list {
+			l.Printf("%s\n", e)
+			if max > 0 && i == max {
+				l.Print("too many errors")
+				return
+			}
+		}
+	} else if err != nil {
+		l.Printf("%s\n", err)
+	}
+}