Looks like this patch caused some tests to fail according to Sasha (avtikhon).
I made another commit fixing the issue.
 
----------------------------------------------------------------------
Previous commit that was merged into master did not do
proper clean-up. That caused occasional failures of other
tests, all with the same error saying user 'guest' already
had access on universe.
 
Closes #714
---
Branch:
https://github.com/tarantool/tarantool/compare/eljashm/gh-714-box-schema-user-grant-invalid-error  

 test/box/access.result   | 18 ++++++++++++------
 test/box/access.test.lua | 10 ++++++----
 2 files changed, 18 insertions(+), 10 deletions(-)
diff --git a/test/box/access.result b/test/box/access.result
index b454d0eaa..e351eaf2c 100644
--- a/test/box/access.result
+++ b/test/box/access.result
@@ -2113,23 +2113,29 @@ box.space._priv:delete{1, 'universe', 0}
 -- i.e. error on universally granted privileges shouldn't
 -- include any redundant details and/or symbols.
 --
-box.schema.user.grant('guest', 'read,write,execute', 'universe')
+box.schema.user.create('grantee')
 ---
 ...
-box.schema.user.grant('guest', 'read,write,execute', 'universe')
+box.schema.user.grant('grantee', 'read,write,execute', 'universe')
 ---
-- error: User 'guest' already has read,write,execute access on universe
+...
+box.schema.user.grant('grantee', 'read,write,execute', 'universe')
+---
+- error: User 'grantee' already has read,write,execute access on universe
 ...
 -- Expected behavior of grant() error shouldn't change otherwise.
 sp = box.schema.create_space('not_universe')
 ---
 ...
-box.schema.user.grant('guest', 'read,write,execute', 'space', 'not_universe')
+box.schema.user.grant('grantee', 'read,write,execute', 'space', 'not_universe')
+---
+...
+box.schema.user.grant('grantee', 'read,write,execute', 'space', 'not_universe')
 ---
+- error: User 'grantee' already has read,write,execute access on space 'not_universe'
 ...
-box.schema.user.grant('guest', 'read,write,execute', 'space', 'not_universe')
+box.schema.user.drop('grantee')
 ---
-- error: User 'guest' already has read,write,execute access on space 'not_universe'
 ...
 sp:drop()
 ---
diff --git a/test/box/access.test.lua b/test/box/access.test.lua
index 387c8b06b..6be558247 100644
--- a/test/box/access.test.lua
+++ b/test/box/access.test.lua
@@ -812,11 +812,13 @@ box.space._priv:delete{1, 'universe', 0}
 -- i.e. error on universally granted privileges shouldn't
 -- include any redundant details and/or symbols.
 --
-box.schema.user.grant('guest', 'read,write,execute', 'universe')
-box.schema.user.grant('guest', 'read,write,execute', 'universe')
+box.schema.user.create('grantee')
+box.schema.user.grant('grantee', 'read,write,execute', 'universe')
+box.schema.user.grant('grantee', 'read,write,execute', 'universe')
 
 -- Expected behavior of grant() error shouldn't change otherwise.
 sp = box.schema.create_space('not_universe')
-box.schema.user.grant('guest', 'read,write,execute', 'space', 'not_universe')
-box.schema.user.grant('guest', 'read,write,execute', 'space', 'not_universe')
+box.schema.user.grant('grantee', 'read,write,execute', 'space', 'not_universe')
+box.schema.user.grant('grantee', 'read,write,execute', 'space', 'not_universe')
+box.schema.user.drop('grantee')
 sp:drop()
-- 
2.24.0 
>Среда, 26 февраля 2020, 1:20 +03:00 от Kirill Yukhin < kyukhin@tarantool.org >:
> 
>Hello,
>
>On 19 фев 19:35, Maria Khaydich wrote:
>>
>> Error message on granted privileges was not flexible and
>> did not distinguish between universal or any other priviliges
>> leaving either 'nil' or simply '' at the end.
>>  
>> Closes #714
>> ----------------------------------------------------------------------
>> Issue:
>>  https://github.com/tarantool/tarantool/issues/714  
>> Branch:
>>  https://github.com/tarantool/tarantool/tree/eljashm/gh-714-box-schema-user-grant-invalid-error  
>I've checked your patch into master.
>
>--
>Regards, Kirill Yukhin 
 
 
--
Maria Khaydich