buttonLabel -> Button label for consistency

lib/browser/api/dialog.js

@@ -78,7 +78,7 @@ module.exports = {
     if (options.buttonLabel == null) {
       options.buttonLabel = ''
     } else if (typeof options.buttonLabel !== 'string') {
-      throw new TypeError('buttonLabel must be a string')
+      throw new TypeError('Button label must be a string')
     }
     if (options.defaultPath == null) {
       options.defaultPath = ''
@@ -111,7 +111,7 @@ module.exports = {
     if (options.buttonLabel == null) {
       options.buttonLabel = ''
     } else if (typeof options.buttonLabel !== 'string') {
-      throw new TypeError('buttonLabel must be a string')
+      throw new TypeError('Button label must be a string')
     }
     if (options.defaultPath == null) {
       options.defaultPath = ''

spec/api-dialog-spec.js

@@ -14,7 +14,7 @@ describe('dialog module', () => {
 
       assert.throws(() => {
         dialog.showOpenDialog({buttonLabel: []})
-      }, /buttonLabel must be a string/)
+      }, /Button label must be a string/)
 
       assert.throws(() => {
         dialog.showOpenDialog({defaultPath: {}})
@@ -30,7 +30,7 @@ describe('dialog module', () => {
 
       assert.throws(() => {
         dialog.showSaveDialog({buttonLabel: []})
-      }, /buttonLabel must be a string/)
+      }, /Button label must be a string/)
 
       assert.throws(() => {
         dialog.showSaveDialog({defaultPath: {}})