changed quit() to quit(1) with a more complete error message to handle the...

changed quit() to quit(1) with a more complete error message to handle the (common) case when the BINARY_C environment variable is not set. With quit(), the return code is 0 so pip just gives a confusing "oserror". With quit(1), an error is raised and the output sent to the terminal.

setup.py

@@ -87,8 +87,8 @@ if not GSL_DIR:
     )
 BINARY_C_DIR = os.getenv("BINARY_C", None)
 if not BINARY_C_DIR:
-    print("Error: the BINARY_C environment variable is not set. Aborting setup")
-    quit()
+    print("\n\n****\n**** Error: the BINARY_C environment variable is not set.\n**** This environment variable should point to the root of your binary_c\n**** installation (i.e. the directory you acquired from the repository).\n**** Aborting setup.\n****\n\n")
+    quit(1)
 
 # TODO: write code to know exact parent directory of this file.
 CWD = os.getcwd()