fix(cli): add check for required files

After failing to deploy the sunpath app I realized that we probably want to make sure the DockerFile and app.py exist in the folder before scheduling it for deployment.

Also added some extra note to docstring and printed out some extra messages to let the user know where to check the app after the deployment.

pollination_apps/cli/__init__.py

@@ -2,6 +2,7 @@
 from pathlib import Path
 
 import click
+from click import ClickException
 from slugify.slugify import slugify
 
 from ..env import Environment
@@ -18,6 +19,12 @@
 @click.pass_context
 def main(ctx: click.Context):
     """ The Pollination Apps CLI
+
+    Use this CLI to create a new app and deploy it to Pollination.
+
+    Try ``pollination-apps new`` and follow the directions for developing the app
+    and deploying your first app to Pollination.
+
     """
     if ctx.invoked_subcommand is None:
         with open(os.path.join(MODULE_PATH, 'assets/art.txt'), 'r') as f:
@@ -92,6 +99,12 @@ def deploy(path, owner, name, tag, message, environment, public):
     if name is None:
         name = path.name
 
+    for required_file in ('Dockerfile', 'app.py'):
+        if not path.joinpath(required_file).is_file():
+            raise ClickException(
+                f'Application folder is missing a required file: {required_file}'
+            )
+
     slug = slugify(name)
 
     if tag is None:
@@ -114,6 +127,15 @@ def deploy(path, owner, name, tag, message, environment, public):
         path=path,
     )
 
+    base_url = 'https://app.staging.pollination.cloud' if environment == 'staging' \
+        else 'https://app.pollination.cloud'
+
+    click.echo(
+        f'\nCongrats! {name} is successfully scheduled for deployment.\n'
+        'It can take a few minutes before the new version of the app is deployed to '
+        'Pollination. You can check the app at this URL: '
+        f'{base_url}/{owner}/applications/{slug}'
+    )
 
 @main.command('new')
 def new():