add(error): introduce zterm.Error containing all zterm errors with their corresponding description

src/zterm.zig

@@ -8,6 +8,7 @@ pub const input = @import("input.zig");
 pub const testing = @import("testing.zig");
 
 pub const App = @import("app.zig").App;
+pub const Error = @import("error.zig").Error;
 // App also exports further types once initialized with the user events at compile time:
 // `App.Container`
 // `App.Element`
@@ -31,6 +32,7 @@ test {
     _ = @import("terminal.zig");
     _ = @import("container.zig");
     _ = @import("queue.zig");
+    _ = @import("error.zig");
 
     _ = color;
     _ = size;