Validate absolute python paths

This makes it easier for someone to realize when they have provided an
invalid Python path without missing that message due to the sizable
virtualenv traceback which follows it. I first encountered this when
someone copy-and-pasted the example from the Conda documentation which
has `--python=/path/to/anaconda/python`.

See pypa#1862

pipenv/cli.py

@@ -56,6 +56,18 @@ def setup_verbose(ctx, param, value):
     return value
 
 
+def validate_python_path(ctx, param, value):
+    # Validating the Python path is complicated by accepting a number of
+    # friendly options: the default will be boolean False to enable
+    # autodetection but it may also be a value which will be searched in
+    # the path or an absolute path. To report errors as early as possible
+    # we'll report absolute paths which do not exist:
+    if isinstance(value, (str, bytes)):
+        if os.path.isabs(value) and not os.path.isfile(value):
+            raise click.BadParameter('Expected Python at path %s does not exist' % value)
+    return value
+
+
 @click.group(
     cls=PipenvGroup,
     invoke_without_command=True,
@@ -106,6 +118,7 @@ def setup_verbose(ctx, param, value):
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -263,6 +276,7 @@ def cli(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -382,6 +396,7 @@ def install(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -453,6 +468,7 @@ def uninstall(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -527,6 +543,7 @@ def lock(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -594,6 +611,7 @@ def shell(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 def run(command, args, three=None, python=False):
@@ -616,6 +634,7 @@ def run(command, args, three=None, python=False):
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -655,6 +674,7 @@ def check(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(
@@ -819,6 +839,7 @@ def graph(bare=False, json=False, reverse=False):
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.argument('module', nargs=1)
@@ -874,6 +895,7 @@ def run_open(module, three=None, python=None):
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option('--bare', is_flag=True, default=False, help="Minimal output.")
@@ -940,6 +962,7 @@ def sync(
     '--python',
     default=False,
     nargs=1,
+    callback=validate_python_path,
     help="Specify which version of Python virtualenv should use.",
 )
 @click.option(