git-gui: Provide fatal error if library is unavailable

If we cannot locate our git-gui library directory, or we find it
but the tclIndex file is not present there (or it is present but
is not something we are allowed to read) the user cannot use the
application.  Rather than silently ignoring the errors related to
the tclIndex file being unavailable we report them up front and
display to the user why we cannot start.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -50,26 +50,33 @@ if {$oguirel eq {1}} {
 } elseif {[string match @@* $oguirel]} {
 	set oguilib [file join [file dirname [file normalize $argv0]] lib]
 }
+
 set idx [file join $oguilib tclIndex]
-catch {
-	set fd [open $idx r]
-	if {[gets $fd] eq {# Autogenerated by git-gui Makefile}} {
-		set idx [list]
-		while {[gets $fd n] >= 0} {
-			if {$n ne {} && ![string match #* $n]} {
-				lappend idx $n
-			}
-		}
-	} else {
-		set idx {}
-	}
-	close $fd
+if {[catch {set fd [open $idx r]} err]} {
+	catch {wm withdraw .}
+	tk_messageBox \
+		-icon error \
+		-type ok \
+		-title "git-gui: fatal error" \
+		-message $err
+	exit 1
 }
+if {[gets $fd] eq {# Autogenerated by git-gui Makefile}} {
+	set idx [list]
+	while {[gets $fd n] >= 0} {
+		if {$n ne {} && ![string match #* $n]} {
+			lappend idx $n
+		}
+	}
+} else {
+	set idx {}
+}
+close $fd
+
 if {$idx ne {}} {
 	set loaded [list]
 	foreach p $idx {
 		if {[lsearch -exact $loaded $p] >= 0} continue
-		puts $p
 		source [file join $oguilib $p]
 		lappend loaded $p
 	}