fix(linux): a refusal to start is shown, not written to nobody

fatalBox off Windows was a single println. That goes to stderr, and a
binary started from a file manager or a .desktop launcher has no stderr
anyone will read — so a carefully worded refusal ("the folder OpsLog
keeps everything in cannot be written to…") reached the operator as "I
click it and nothing happens", which is the least useful thing a program
can say.

It now opens a dialog through zenity or kdialog when either is present —
both ship with every distribution's desktop task — and writes to stderr
either way. Neither is required: the startup log already carries the
same text, and nothing here may stop OpsLog from exiting.

fatalbox_other.go

@@ -2,5 +2,38 @@
 
 package main
 
-// fatalBox is Windows-only; elsewhere the terminal carries the message.
+import (
-func fatalBox(title, text string) { println(title + ": " + text) }
+	"os"
+	"os/exec"
+)
+
+// fatalBox says why OpsLog is not starting, as visibly as the desktop allows.
+//
+// It used to be one println. That goes to stderr, and a binary started from a
+// file manager or a .desktop launcher has no stderr anybody will ever read — so
+// the refusal that was carefully worded arrived as "I click it and nothing
+// happens", which is the least useful thing a program can say.
+//
+// So: a real dialog when the desktop has one of the two tools that every
+// distribution ships with its desktop task, and stderr regardless. Neither is
+// required — the startup log has the same text, and nothing here is allowed to
+// stop OpsLog from exiting.
+func fatalBox(title, text string) {
+	os.Stderr.WriteString(title + ": " + text + "\n")
+	for _, c := range [][]string{
+		{"zenity", "--error", "--no-wrap", "--title", title, "--text", text},
+		{"kdialog", "--title", title, "--error", text},
+		// Neither is a hard requirement, and on a headless box there is nothing
+		// to show a dialog on anyway.
+	} {
+		if _, err := exec.LookPath(c[0]); err != nil {
+			continue
+		}
+		cmd := exec.Command(c[0], c[1:]...)
+		if err := cmd.Start(); err != nil {
+			continue
+		}
+		_ = cmd.Wait()
+		return
+	}
+}