Change conversion of box.error to string in error message assertions

Currently, error message assertions convert `box.error` to string using
`tostring`. `box.error`'s `__tostring` metamethod is currently
equivalent to the `message` field of `box.error`. In scope of
tarantool/tarantool#9105, we are going to increase the verbosity of
`box.error` `__tostring` metamethod, and it will no longer be
convenient for testing the error message. Let's replace `tostring(error)`
with `error.message` usag to retain the old behaviour.

Needed for tarantool/tarantool#9105

Author: CuriousGeorgiy

luatest/assertions.lua

@@ -468,7 +468,7 @@ local function _assert_error_msg_equals(stripFileAndLine, expectedMsg, func, ...
     end
     if type(expectedMsg) == "string" and type(error_msg) ~= "string" then
         -- table are converted to string automatically
-        error_msg = tostring(error_msg)
+        error_msg = error_msg.message
     end
     local differ = false
     if stripFileAndLine then
@@ -533,7 +533,7 @@ function M.assert_error_msg_contains(expected_partial, fn, ...)
         failure(failure_message, nil, 2)
     end
     if type(error_msg) ~= "string" then
-        error_msg = tostring(error_msg)
+        error_msg = error_msg.message
     end
     if not string.find(error_msg, expected_partial, nil, true) then
         error_msg, expected_partial = prettystr_pairs(error_msg, expected_partial)
@@ -555,7 +555,7 @@ function M.assert_error_msg_matches(pattern, fn, ...)
         failure(failure_message, nil, 2)
     end
     if type(error_msg) ~= "string" then
-        error_msg = tostring(error_msg)
+        error_msg = error_msg.message
     end
     if not str_match(error_msg, pattern) then
         pattern, error_msg = prettystr_pairs(pattern, error_msg)