[CE-431] Fixed incorrect error display

Error text retrieval changed from `err.err` to `tostring(err)` based on the following reasons:

1. If `err` happens to be a string, the result will also be a string
2. If `err` is an instance of `vshard.error`, it has a `tostring` operator that converts the structure into a JSON string
3. If `err` is an instance of the generic `errors` module, it also provides a `tostring` method

(cherry picked from commit cf7d963bea98a1f21a673531c88cec98dcaf58a1)

Author: a1div0

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 * `crud.schema` no longer returns system space `_gc_consumers` with Tarantool 3.2+.
 * Tests of `schema` with Tarantool 3.2+.
+* Fixed bad error handling for method `call.single`
 
 ## [1.5.2] - 20-05-24
 

crud/common/call.lua

@@ -198,7 +198,7 @@ function call.single(vshard_router, bucket_id, func_name, func_args, opts)
 
     local replicaset, err = vshard_router:route(bucket_id)
     if err ~= nil then
-        return nil, CallError:new("Failed to get router replicaset: %s", err.err)
+        return nil, CallError:new("Failed to get router replicaset: %s", tostring(err))
     end
 
     local timeout = opts.timeout or const.DEFAULT_VSHARD_CALL_TIMEOUT

test/integration/custom_bucket_id_test.lua

@@ -555,3 +555,17 @@ pgroup.test_select = function(g)
     -- tuple is found
     t.assert_equals(#result.rows, 1)
 end
+
+pgroup.test_non_existent_bucket_id = function(g)
+    local _, err = g.router:call('crud.insert', {
+        'customers', {1, box.NULL, 'Maria', 23}, {bucket_id = 999999}
+    })
+    t.assert_equals(type(err), 'table')
+    t.assert_equals(err.class_name, 'InsertError')
+    t.assert_str_contains(
+        err.err,
+        'Failed to call insert on storage-side: CallError: Failed to get router replicaset: '
+        .. '{"bucket_id":999999,"code":9,"type":"ShardingError","message":"Bucket 999999 cannot'
+        .. ' be found. Is rebalancing in progress?","name":"NO_ROUTE_TO_BUCKET"}'
+    )
+end