harden GDB plugin against missing file paths, allow plugin to compute relative path

package.json

@@ -76,7 +76,7 @@
         "c9.ide.find": "#a2dfc3e306",
         "c9.ide.find.infiles": "#488db22ee1",
         "c9.ide.find.replace": "#47a84af707",
-        "c9.ide.run.debug": "#8ae86ca02f",
+        "c9.ide.run.debug": "#651451a7c2",
         "c9.automate": "#47e2c429c9",
         "c9.ide.ace.emmet": "#6dc4585e02",
         "c9.ide.ace.gotoline": "#d33220b1e0",
@@ -97,7 +97,6 @@
         "c9.ide.installer": "#b2e4ba0a92",
         "c9.ide.language.python": "#9fba572b1a",
         "c9.ide.language.go": "#6ce1c7a7ef",
-        "c9.ide.mount": "#b018ee501f",
         "c9.ide.navigate": "#5d5707058c",
         "c9.ide.newresource": "#981a408a7b",
         "c9.ide.openfiles": "#2ae85a9e33",