Invalid Argument errors provide per field details. (#110)

* Invalid Argument errors now return a per field error message.

* Creating an API error struct.

* SDK uses field error details proto.

* Adding tests specifically for handler helper error messages resulting in expected API output.

* Inlining the error checks into the table driven tests.  Use protocmp for assertions to make printing out diffs easier to parse.

* Aggregate the invalid argument fields detected to be wrong.

* Make the returned order of fields deterministic in the error details..

* Fixing formatting issue in errors_test.go.

* Test to confirm unimplemented results in a 405 and not 404.

api/field_error.gen.go

@@ -0,0 +1,36 @@
+// Code generated by "make api"; DO NOT EDIT.
+package api
+
+import (
+	"encoding/json"
+
+	"github.com/fatih/structs"
+
+	"github.com/hashicorp/watchtower/api/internal/strutil"
+)
+
+type FieldError struct {
+	defaultFields []string
+
+	Name        *string `json:"name,omitempty"`
+	Description *string `json:"description,omitempty"`
+}
+
+func (s *FieldError) SetDefault(key string) {
+	s.defaultFields = strutil.AppendIfMissing(s.defaultFields, key)
+}
+
+func (s *FieldError) UnsetDefault(key string) {
+	s.defaultFields = strutil.StrListDelete(s.defaultFields, key)
+}
+
+func (s FieldError) MarshalJSON() ([]byte, error) {
+	m := structs.Map(s)
+	if m == nil {
+		m = make(map[string]interface{})
+	}
+	for _, k := range s.defaultFields {
+		m[k] = nil
+	}
+	return json.Marshal(m)
+}