Moved general error instance to the filters package to be reused

Closes #18

filters/filters.go

@@ -3,6 +3,7 @@ package filters
 import (
 	// import filter packages here:
 
+	"errors"
 	"github.com/zalando/skipper/filters/healthcheck"
 	"github.com/zalando/skipper/filters/humanstxt"
 	"github.com/zalando/skipper/filters/pathrewrite"
@@ -14,6 +15,10 @@ import (
 	"github.com/zalando/skipper/skipper"
 )
 
+var (
+	ErrInvalidFilterParameters = errors.New("Invalid filter parameters")
+)
+
 // takes a registry object and registers the filter spec in the package
 func Register(registry skipper.FilterRegistry) {
 	registry.Add(

filters/flowid/filter.go

@@ -1,7 +1,7 @@
 package flowid
 
 import (
-	"errors"
+	"github.com/zalando/skipper/filters"
 	"github.com/zalando/skipper/skipper"
 )
 
@@ -16,10 +16,6 @@ type flowId struct {
 	flowIdLength  uint8
 }
 
-var (
-	ErrInvalidFilterParameters = errors.New("Invalid filter parameters")
-)
-
 func New(id string, allowOverride bool, len uint8) skipper.Filter {
 	return &flowId{id, allowOverride, len}
 }
@@ -53,15 +49,15 @@ func (this *flowId) MakeFilter(id string, fc skipper.FilterConfig) (skipper.Filt
 		if r, ok := fc[0].(bool); ok {
 			reuseExisting = r
 		} else {
-			return nil, ErrInvalidFilterParameters
+			return nil, filters.ErrInvalidFilterParameters
 		}
 	}
 	var flowIdLength uint8 = defaultLen
 	if len(fc) > 1 {
 		if l, ok := fc[1].(float64); ok && l >= minLength && l <= maxLength {
 			flowIdLength = uint8(l)
 		} else {
-			return nil, ErrInvalidFilterParameters
+			return nil, filters.ErrInvalidFilterParameters
 		}
 	}
 	return New(id, reuseExisting, flowIdLength), nil