Report file and line number on invalid bp position

Fixes #89

lib/v8debugapi.js

@@ -42,7 +42,7 @@
   V8_BREAKPOINT_MISSING: 'Internal error: V8 breakpoint missing',
   V8_BREAKPOINT_DISABLED: 'Internal error: V8 breakpoint externally disabled',
   CAPTURE_BREAKPOINT_DATA: 'Error trying to capture breakpoint data: ',
-  INVALID_LINE_NUMBER: 'Breakpoint line number is invalid'
+  INVALID_LINE_NUMBER: 'Invalid breakpoint position: '
 };
 
 /** @const */ var MODULE_WRAP_PREFIX_LENGTH = require('module').wrap('☃')
@@ -267,7 +267,11 @@ module.exports.create = function(logger_, config_, fileStats_) {
     if (breakpoint.location.line >= fileStats[matchingScript].lines) {
       return setErrorStatusAndCallback(cb, breakpoint,
         StatusMessage.BREAKPOINT_SOURCE_LOCATION,
-        messages.INVALID_LINE_NUMBER);
+        messages.INVALID_LINE_NUMBER + matchingScript + ':' +
+          breakpoint.location.line + '. Loaded script contained ' +
+          fileStats[matchingScript].lines + ' lines. Please ensure' +
+          ' that the breakpoint was set in the same code version as the' +
+          ' deployed source.');
     }
 
     // The breakpoint protobuf message presently doesn't have a column property

test/test-v8debugapi.js

@@ -247,8 +247,10 @@ describe('v8debugapi', function() {
         assert.ok(bp.status);
         assert.ok(bp.status instanceof StatusMessage);
         assert.ok(bp.status.isError);
-        assert(bp.status.description.format ===
-          api.messages.INVALID_LINE_NUMBER);
+        assert(bp.status.description.format.indexOf(
+          api.messages.INVALID_LINE_NUMBER) !== -1);
+        assert(bp.status.description.format.indexOf(
+          'foo.js:500') !== -1);
         done();
       });
     });