nikola init should fail fast on existing target directory
When I run `nikola init --demo .` and fill all the stuff an error is thrown: `FileExistsError: [Errno 17] File exists: '.'`.

The problem is that I have to fill in all the details again. The app should first check the directory and fail fast so that the user can fix the problem and THEN fill in the details.

diff --git a/nikola/plugins/command/init.py b/nikola/plugins/command/init.py
index 0795a9d7e..96fb6e32e 100644
--- a/nikola/plugins/command/init.py
+++ b/nikola/plugins/command/init.py
@@ -221,6 +221,18 @@ def prepare_config(config):
     return p
 
 
+def test_destination(destination, demo=False):
+    """Check if the destination already exists, which can break demo site creation."""
+    # Issue #2214
+    if demo and os.path.exists(destination):
+        LOGGER.warning("The directory {0} already exists, and a new demo site cannot be initialized in an existing directory.".format(destination))
+        LOGGER.warning("Please remove the directory and try again, or use another directory.")
+        LOGGER.info("Hint: If you want to initialize a git repository in this directory, run `git init` in the directory after creating a Nikola site.")
+        return False
+    else:
+        return True
+
+
 class CommandInit(Command):
     """Create a new site."""
 
@@ -277,7 +289,7 @@ class CommandInit(Command):
             makedirs(os.path.join(target, folder))
 
     @staticmethod
-    def ask_questions(target):
+    def ask_questions(target, demo=False):
         """Ask some questions about Nikola."""
         def urlhandler(default, toconf):
             answer = ask('Site URL', 'https://example.com/')
@@ -442,7 +454,7 @@ class CommandInit(Command):
         print("If you do not want to answer and want to go with the defaults instead, simply restart with the `-q` parameter.")
 
         for query, default, toconf, destination in questions:
-            if target and destination == '!target':
+            if target and destination == '!target' and test_destination(target, demo):
                 # Skip the destination question if we know it already
                 pass
             else:
@@ -459,8 +471,9 @@ class CommandInit(Command):
                     if toconf:
                         SAMPLE_CONF[destination] = answer
                     if destination == '!target':
-                        while not answer:
-                            print('    ERROR: you need to specify a target directory.\\n')
+                        while not answer or not test_destination(answer, demo):
+                            if not answer:
+                                print('    ERROR: you need to specify a target directory.\\n')
                             answer = ask(query, default)
                         STORAGE['target'] = answer
 
@@ -476,7 +489,7 @@ class CommandInit(Command):
         except IndexError:
             target = None
         if not options.get('quiet'):
-            st = self.ask_questions(target=target)
+            st = self.ask_questions(target=target, demo=options.get('demo'))
             try:
                 if not target:
                     target = st['target']
@@ -489,11 +502,13 @@ class CommandInit(Command):
 Options:
   -q, --quiet               Do not ask questions about config.
   -d, --demo                Create a site filled with example data.""")
-            return False
+            return 1
         if not options.get('demo'):
             self.create_empty_site(target)
             LOGGER.info('Created empty site at {0}.'.format(target))
         else:
+            if not test_destination(target, True):
+                return 2
             self.copy_sample_site(target)
             LOGGER.info("A new site with example data has been created at "
                         "{0}.".format(target))
['nikola/plugins/command/init.py']