Improve the modern error message + add UUTILS_DIAG to control it

README.md

@@ -45,7 +45,10 @@ are treated as bugs.
 
 Our key objectives include:
 - Matching GNU's output (stdout and error code) exactly
-- Better error messages
+- Better error messages: at a terminal, a parse error is shown as a
+  compiler-style report with a caret under the argument at fault, where GNU
+  prints a single line (see [error diagnostics](docs/src/extensions-errors.md));
+  scripts and pipes still get the plain GNU message
 - Providing comprehensive internationalization support (UTF-8)
 - Improved performances
 - [Extensions](docs/src/extensions.md) when relevant (example: --progress)

docs/src/extensions-errors.css

@@ -0,0 +1,42 @@
+/* Styling for the error reports shown on this page. The colors are the ones
+   the utilities actually emit, so the examples read here the way they do in a
+   terminal; they match the palette used on uutils.org. */
+
+pre.diag {
+    margin: 1rem 0;
+    padding: 0.9rem 1.1rem;
+    border-radius: 0.6em;
+    background: #1e1e2e;
+    color: #cdd6f4;
+    font-size: 0.82rem;
+    line-height: 1.5;
+    overflow-x: auto;
+    white-space: pre;
+}
+
+/* The prompt of the command being run. */
+pre.diag .a-p {
+    color: #89b4fa;
+}
+
+/* ANSI colors as emitted by the utilities: frame, source line, gutter,
+   the span at fault, and the Help: label. */
+pre.diag .a-d {
+    color: #9399b2;
+}
+
+pre.diag .a-s {
+    color: #cdd6f4;
+}
+
+pre.diag .a-f {
+    color: #7f849c;
+}
+
+pre.diag .a-e {
+    color: #f38ba8;
+}
+
+pre.diag .a-h {
+    color: #a6e3a1;
+}