az webapp new throws an exception when it can't detect a project type.

[anthonychu]

Extension name (the extension in question)

webapp

Description of issue (in as much detail as possible)

This should fail gracefully:

bash-4.3# cd /
bash-4.3# ls
appp       bin        etc        lib        media      proc       run        srv        tmp        var
azure-cli  dev        home       linuxrc    mnt        root       sbin       sys        usr
bash-4.3# az webapp new -n foo
list index out of range
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/knack/cli.py", line 194, in invoke
    cmd_result = self.invocation.execute(args)
  File "/usr/local/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 331, in execute
    six.reraise(*sys.exc_info())
  File "/usr/local/lib/python3.6/site-packages/six.py", line 693, in reraise
    raise value
  File "/usr/local/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 304, in execute
    result = cmd(params)
  File "/usr/local/lib/python3.6/site-packages/azure/cli/core/commands/__init__.py", line 168, in __call__
    return super(AzCliCommand, self).__call__(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/knack/commands.py", line 109, in __call__
    return self.handler(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/azure/cli/core/__init__.py", line 348, in default_command_handler
    result = op(**command_args)
  File "/root/.azure/cliextensions/webapp/azext_webapp/custom.py", line 50, in create_deploy_webapp
    lang_details = get_lang_from_content(src_dir)
  File "/root/.azure/cliextensions/webapp/azext_webapp/create_util.py", line 113, in get_lang_from_content
    package_netcore_file = os.path.join(src_path, glob.glob("*.csproj")[0])
IndexError: list index out of range
bash-4.3# 

Steps to reproduce: run az webapp new -n foo in a non-empty directory that does not contain a package.json or a *.csproj file.