Check lockfile exists on sync, and don't update it

Matching the behaviour mentioned in:
#1463 (comment)

pipenv/core.py

@@ -2492,19 +2492,30 @@ def do_sync(
     unused=False,
     sequential=False,
 ):
-    requirements_dir = TemporaryDirectory(
-        suffix='-requirements', prefix='pipenv-'
-    )
+    # The lock file need to exist because sync won't write to it.
+    if not project.lockfile_exists:
+        click.echo(
+            '{0}: Pipfile.lock is missing! You need to run {1} first.'.format(
+                crayons.red('Error', bold=True),
+                crayons.red('$ pipenv lock', bold=True),
+            ),
+            err=True,
+        )
+        sys.exit(1)
+
     # Ensure that virtualenv is available.
     ensure_project(three=three, python=python, validate=False)
-    concurrent = (not sequential)
-    ensure_lockfile()
+
     # Install everything.
+    requirements_dir = TemporaryDirectory(
+        suffix='-requirements', prefix='pipenv-'
+    )
     do_init(
         dev=dev,
         verbose=verbose,
-        concurrent=concurrent,
+        concurrent=(not sequential),
         requirements_dir=requirements_dir,
+        ignore_pipfile=True,    # Don't check if Pipfile and lock match.
     )
     requirements_dir.cleanup()
     click.echo(crayons.green('All dependencies are now up-to-date!'))

tests/integration/test_sync.py

@@ -0,0 +1,42 @@
+import pytest
+
+
+@pytest.mark.sync
+def test_sync_error_without_lockfile(PipenvInstance, pypi):
+    with PipenvInstance(pypi=pypi) as p:
+        with open(p.pipfile_path, 'w') as f:
+            f.write("""
+[packages]
+            """.strip())
+
+        c = p.pipenv('sync')
+        assert c.return_code != 0
+        assert 'Pipfile.lock is missing!' in c.err
+
+
+@pytest.mark.sync
+@pytest.mark.lock
+def test_sync_should_not_lock(PipenvInstance, pypi):
+    """Sync should not touch the lock file, even if Pipfile is changed.
+    """
+    with PipenvInstance(pypi=pypi) as p:
+        with open(p.pipfile_path, 'w') as f:
+            f.write("""
+[packages]
+            """.strip())
+
+        # Perform initial lock.
+        c = p.pipenv('lock')
+        assert c.return_code == 0
+        lockfile_content = p.lockfile
+        assert lockfile_content
+
+        # Make sure sync does not trigger lockfile update.
+        with open(p.pipfile_path, 'w') as f:
+            f.write("""
+[packages]
+six = "*"
+            """.strip())
+        c = p.pipenv('sync')
+        assert c.return_code == 0
+        assert lockfile_content == p.lockfile