Avoid breaking the build of yast2.rpm (bsc#1130822)

If /usr/share/YaST2/locale does not exist (eg. during the building of
yast2.rpm) FastGettext will complain. So in that case bail out of
`textdomain` and `_`+friends.

Here's how it broke without this fix:

+Log	No textdomain configured in Yast::CommandLineClass, cannot translate "Print the help for this module"

The log line *appears* because it is now reported at CommandLine.rb
(previously i18n.rb) which matches the "testedfiles" filter of the
testsuite.

The error *happens* the "textdomain" method returns early before
remembering its argument.

src/ruby/yast/i18n.rb

@@ -53,10 +53,12 @@ def textdomain(domain) # usually without brackets like textdomain "example"
     def _(str)
       # no textdomain configured yet
       if !@my_textdomain
-        msg = "No textdomain configured in #{self.class}, " \
-              "cannot translate #{str.inspect}"
-        raise msg if ENV["Y2STRICTTEXTDOMAIN"]
-        Yast.y2warning(1, "%1", msg) # skip 1 frame
+        if File.exist?(LOCALE_DIR)
+          msg = "No textdomain configured in #{self.class}, " \
+                "cannot translate #{str.inspect}"
+          raise msg if ENV["Y2STRICTTEXTDOMAIN"]
+          Yast.y2warning(1, "%1", msg) # skip 1 frame
+        end
         return str.freeze
       end
 
@@ -123,11 +125,13 @@ def Nn_(*keys)
     def n_(singular, plural, num)
       # no textdomain configured yet
       if !@my_textdomain
-        # it's enough to log just the singular form
-        msg = "No textdomain configured in #{self.class}, " \
-              "cannot translate #{singular.inspect}"
-        raise msg if ENV["Y2STRICTTEXTDOMAIN"]
-        Yast.y2warning(1, "%1", msg) # skip 1 frame
+        if File.exist?(LOCALE_DIR)
+          # it's enough to log just the singular form
+          msg = "No textdomain configured in #{self.class}, " \
+                "cannot translate #{singular.inspect}"
+          raise msg if ENV["Y2STRICTTEXTDOMAIN"]
+          Yast.y2warning(1, "%1", msg) # skip 1 frame
+        end
         return fallback_n_(singular, plural, num)
       end