config: don't crash when there is an error parsing interpolation

[GH-282]

config/expr_lex.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"bytes"
+	"fmt"
 	"log"
 	"unicode"
 	"unicode/utf8"
@@ -13,6 +14,8 @@ const lexEOF = 0
 // The parser uses the type <prefix>Lex as a lexer.  It must provide
 // the methods Lex(*<prefix>SymType) int and Error(string).
 type exprLex struct {
+	Err error
+
 	input string
 	pos   int
 	width int
@@ -127,5 +130,5 @@ func (x *exprLex) backup() {
 
 // The parser calls this method on a parse error.
 func (x *exprLex) Error(s string) {
-	log.Printf("parse error: %s", s)
+	x.Err = fmt.Errorf("parse error: %s", s)
 }

config/expr_parse.go

@@ -21,12 +21,18 @@ func ExprParse(v string) (Interpolation, error) {
 	exprResult = nil
 
 	// Parse
-	exprParse(&exprLex{input: v})
+	lex := &exprLex{input: v}
+	exprParse(lex)
 
 	// Build up the errors
 	var err error
+	if lex.Err != nil {
+		err = multierror.ErrorAppend(err, lex.Err)
+	}
 	if len(exprErrors) > 0 {
-		err = &multierror.Error{Errors: exprErrors}
+		err = multierror.ErrorAppend(err, exprErrors...)
+	}
+	if err != nil {
 		exprResult = nil
 	}