fix: verify no errors during npm install (#132)

The case:
- We try to install nativescript-dev-sass@next which is not available.
- Before this fix it will fail silently and project will use some old sass plugin version (or even it will hav eno sass plugin and it will fail)
- Now we check if npm install is correct
- This check may lead to failing tests where we install invalid version, but this will actually make our tests valid.

Author: dtopuzov

core/npm/npm.py

@@ -40,7 +40,9 @@ def pack(folder, output_file):
     def install(package='', option='', folder=None, log_level=CommandLogLevel.FULL):
         if package is None:
             raise NameError('Package can not be None.')
-        return Npm.__run_npm_command('i {0} {1}'.format(package, option), folder=folder, log_level=log_level)
+        command = 'i {0} {1}'.format(package, option)
+        output = Npm.__run_npm_command(command, folder=folder, log_level=log_level)
+        assert "ERR!" not in output, "`npm " + command + "` failed with: \n" + output
 
     @staticmethod
     def uninstall(package, option='', folder=None, log_level=CommandLogLevel.FULL):