refactor: extract base exporter, add sentinel errors, document unused fields

internal/apperrors/errors.go

@@ -0,0 +1,28 @@
+// Package apperrors provides sentinel errors for the articulate-parser application.
+package apperrors
+
+import "errors"
+
+// Export errors.
+var (
+	// ErrUnsupportedFormat is returned when an unknown export format is requested.
+	ErrUnsupportedFormat = errors.New("unsupported export format")
+)
+
+// URI parsing errors.
+var (
+	// ErrInvalidURI is returned when a URI cannot be parsed.
+	ErrInvalidURI = errors.New("invalid URI")
+
+	// ErrInvalidDomain is returned when the URI domain is not an Articulate Rise domain.
+	ErrInvalidDomain = errors.New("invalid domain for Articulate Rise URI")
+
+	// ErrNoShareID is returned when a share ID cannot be extracted from the URI.
+	ErrNoShareID = errors.New("could not extract share ID from URI")
+)
+
+// Operation errors.
+var (
+	// ErrOperationCanceled is returned when an operation is canceled via context.
+	ErrOperationCanceled = errors.New("operation canceled")
+)