Use-as-a-library footgun: Diagnostics.errors is only set during rendering

[squeek502]

Diagnostics.errors is only modified within Diagnostics.renderMessages:

arocc/src/aro/Diagnostics.zig

Line 349 in 7dd1575

comp.diagnostics.errors += errors;

---

Driver always calls render before checking errors, so it's not hit by this:

arocc/src/aro/Driver.zig

Lines 621 to 626 in 7dd1575

	d.renderErrors();
	if (d.comp.diagnostics.errors != 0) {
	if (fast_exit) std.process.exit(1); // Not linking, no need for cleanup.
	return;
	}

but I only want to render errors if there was an actual error, so I attempted this:

    try pp.preprocessSources(&.{ source, builtin_macros, user_macros });

    if (comp.diagnostics.errors != 0) {
        return error.FatalError;
    }

and was confused as to why errors was still 0 after a preprocessing error had definitely occurred.