lib+test: unify errors to Hare named-void tagged-union

Five tags from ref/hare/errors/common.ha — invalid, noaccess, noentry,
exists, unsupported — all !void. lib/io keeps eof/closed as plain void
(no Hare analogue for ww's singleton-style done) and adds underread.
Drops errors.equal/isnil and the old str-sentinel surface.

lib/errors/errors.ww

@@ -1,30 +1,25 @@
-// errors — error type (a string) and a few sentinels. Plan 9 model:
-// the empty string means OK, a non-empty string is the message.
+// errors — domain-agnostic error types. Mirrors ref/hare/errors/.
+//
+// Named-void tagged-union variants, so `(T | errors.invalid | ...)`
+// composes with every other module's error surface at the tag level.
+// Per-domain errors (io.eof, io.closed, io.underread, strconv.overflow,
+// ...) live in their own modules; this module ships only the generic
+// conditions Hare's errors:: exports.
+//
+// Subset shipped today; add more from Hare's errors/common.ha as callers
+// need them.
 
-type error = str;
+// A function was called with an invalid combination of arguments.
+export type invalid = !void;
 
-def eof:        error = "eof";
-def underread:  error = "short read";
-def underwrite: error = "short write";
-def closed:     error = "closed";
-def invalid:    error = "invalid argument";
-def noaccess:   error = "permission denied";
-def noentry:    error = "not found";
-def exists:     error = "already exists";
+// The user does not have permission to use this resource.
+export type noaccess = !void;
 
-export fn isnil(e: error) bool = {
-	return e.len == 0;
-};
+// An entry was requested which does not exist.
+export type noentry = !void;
 
-// equal — compare an error against a sentinel (or any other error).
-// Pure byte equality. (Was `errors.is` before `is` became a keyword
-// for tagged-union type-tests; rename matches bytes.equal / strings.equal.)
-export fn equal(e: error, want: error) bool = {
-	if (e.len != want.len) { return false; };
-	let i: i32 = 0;
-	for (i < e.len) {
-		if (e[i] != want[i]) { return false; };
-		i += 1;
-	};
-	return true;
-};
+// An attempt was made to create a resource which already exists.
+export type exists = !void;
+
+// The requested operation is not supported.
+export type unsupported = !void;