Table source plugin - validate the sql table before executing the query

Problem/Motivation

If the table does not exist in the source database, the migration will fail with the database exception:

[error]  Could not retrieve source count from sql: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'source.foo' doesn't exist: SELECT COUNT(*) AS "expression"
FROM
(SELECT 1 AS "expression"
FROM
"foo" "t") "subquery"; Array
(
)

My specific use-case: custom migration that was executed on multiple websites, where the data structure is a bit different.

Steps to reproduce

1) Create a migration with Table source plugin;
2) Specify not existing sql table;
3) Try running the migration;

Proposed resolution

Implement ::checkRequirements method in Table source plugin.

Remaining tasks

Submit a merge request, review, commit.