cython requirement checks: having central place for required version …

…and display cython import error for easier debugging.

setup.py

@@ -10,13 +10,15 @@
 
 """
 
+required_cython_version = '0.28'
+
 no_cython_message = """
-You need to have Cython (>= 0.28) installed to build the snappy
+You need to have Cython (>= %s) installed to build the snappy
 module since you're missing the autogenerated C/C++ files, e.g.
 
-  sudo python -m pip install "cython>=0.28"
+  sudo python -m pip install "cython>=%s"
 
-"""
+""" % (required_cython_version, required_cython_version)
 
 no_sphinx_message = """
 You need to have Sphinx installed to rebuild the
@@ -288,16 +290,23 @@ def run(self):
     from Cython.Build import cythonize
     from Cython import __version__ as cython_version
     have_cython = True
-except ImportError:
+except ImportError as e:
     have_cython = False
+    cython_import_error = e
 
 def replace_ext(file, new_ext):
     root, ext = os.path.splitext(file)
     return root + '.' + new_ext
 
+def split_version(s : str):
+    return [int(x) for x in s.split('.')]
+
 if have_cython:
-    if [int(x) for x in cython_version.split('.')[:2]] < [0, 28]:
-        raise ImportError
+    if split_version(cython_version) < split_version(required_cython_version):
+        raise ImportError(
+            'Wrong cython version installed. '
+            'Required version: %s. Installed version: %s.' % (
+                required_cython_version, cython_version))
 
     if 'clean' not in sys.argv:
         cython_sources = [file for file in cython_sources if exists(file)]
@@ -311,8 +320,10 @@ def replace_ext(file, new_ext):
     targets += [replace_ext(file, 'cpp') for file in cython_cpp_sources]
     for file in targets:
         if not exists(file):
-            raise ImportError(no_cython_message +
-                              'Missing Cythoned file: ' + file)
+            raise ImportError(
+                no_cython_message +
+                'Missing Cythoned file: ' + file +
+                '\n[Cython import error: %r]' % cython_import_error)
 
 # We check manually which object files need to be rebuilt; distutils
 # is overly cautious and always rebuilds everything, which makes