Validate target type for all option fields, not just features (#279)

The code to verify that a feature field was only used on particular kind
of element was features-specific. However, in `protoc`, it is not
features-specific and applies to _all_ option fields that have a
`targets` option defined. So this updates protocompile to behave the
same way.

This PR is a bit complicated because I had to change the error handling
of the `*interpreter.fieldValue` method (and the `setOptionField` method
that called it, as well as numerous methods that it used). It previously
either returned a value or an error. If it returned an error, it would
be reported by the caller. But now, these places handle the errors, to
allow reporting of multiple errors for the same option value. This change
in behavior is the reason for most of the churn/change in the code.

The change to just the target type validation is actually a net simplification.

linker/linker_test.go

@@ -1379,7 +1379,8 @@ func TestLinkerValidation(t *testing.T) {
 					  };
 					}`,
 			},
-			expectedErr: `foo.proto:9:6: message foo.bar.Baz: option (foo.bar.any): any type references cannot be repeated or mixed with other fields`,
+			expectedErr: `foo.proto:9:6: message foo.bar.Baz: option (foo.bar.any): any type references cannot be repeated or mixed with other fields` +
+				` && foo.proto:13:6: message foo.bar.Baz: option (foo.bar.any): any type references cannot be repeated or mixed with other fields`,
 		},
 		"failure_scope_type_name": {
 			input: map[string]string{
@@ -2444,7 +2445,7 @@ func TestLinkerValidation(t *testing.T) {
 					}
 				`,
 			},
-			expectedErr: `test.proto:3:27: feature "enum_type" is allowed on [enum,file], not on field`,
+			expectedErr: `test.proto:3:27: field "google.protobuf.FeatureSet.enum_type" is allowed on [enum,file], not on field`,
 		},
 		"failure_editions_feature_on_wrong_target_type_msg_literal": {
 			input: map[string]string{
@@ -2457,7 +2458,7 @@ func TestLinkerValidation(t *testing.T) {
 					}
 				`,
 			},
-			expectedErr: `test.proto:4:5: feature "enum_type" is allowed on [enum,file], not on field`,
+			expectedErr: `test.proto:4:5: field "google.protobuf.FeatureSet.enum_type" is allowed on [enum,file], not on field`,
 		},
 		"failure_proto3_enum_zero_value": {
 			input: map[string]string{