Have krb5.init_context() raise an exception on error. (#22)

init_context() currently just returns a null pointer if the C routine
fails, which is unlike the norm in this module and I assume is an
oversight. Typically this will fail if there's a syntax error in the
libkrb5 configuration (/etc/krb5.conf, or whatever is read). A program
I wrote segfaulted in this situation, because I then passed the null
context to other routines. With this change, we get instead:

In [2]: ctx = krb5.init_context()
---------------------------------------------------------------------------
Krb5Error                                 Traceback (most recent call last)
...
Krb5Error: Included profile file could not be read -1429577697

src/krb5/_context.pyx

@@ -60,8 +60,12 @@ cdef class Context:
 
 
 def init_context() -> Context:
+    cdef krb5_error_code = 0
     context = Context()
-    krb5_init_context(&context.raw)
+
+    err = krb5_init_context(&context.raw)
+    if err:
+        raise Krb5Error(context, err)
 
     return context