Match spec error code "invalid context entry".

Tests were also using "invalid context member" and have been updated to
match the spec.

Author: davidlehn

lib/context.js

@@ -274,7 +274,7 @@ api.process = async ({
           'Invalid JSON-LD syntax; @import not compatible with ' +
           activeCtx.processingMode,
           'jsonld.SyntaxError',
-          {code: 'invalid context member', context: ctx});
+          {code: 'invalid context entry', context: ctx});
       }
       if(!_isString(value)) {
         throw new JsonLdError(
@@ -300,7 +300,7 @@ api.process = async ({
         throw new JsonLdError(
           'Invalid JSON-LD syntax; imported context must not include @import.',
           'jsonld.SyntaxError',
-          {code: 'invalid context member', context: localCtx});
+          {code: 'invalid context entry', context: localCtx});
       }
 
       // merge ctx into importCtx and replace rval with the result