throw FatalIOerror instead of FatalError in timeActivateFileUpdate

In timeActivateFileUpdate, the filechecks emit a FatalError if any of the source files are missing on startup. By default this throw/catch is downgraded to a warning unless the user specifies a different error handling. However, since this functionObject is intended as a control mechanism (similar to system/controlDict, for example), it makes more sense for a missing file to be really fatal. Elevating to an IOerror will do this for the default error handling.

cross-ref EP1968