Add a new initialization step to validate IRB.conf's values

[st0012]

Currently, users can only find out that they have set a wrong value for IRB configs when the value is used, with opaque error messages like "comparison of Integer with true failed (TypeError)".

This commit adds a new initialization step to validate the values of some IRB configs, so that users can find out about the wrong values during the initialization of IRB.

Closes #774