Ticket 156: Replace "an error is signalled" with "an implementation-dependent object is raised as if by `raise`"

The following situations are described in draft 1 (and R5RS) with "an error is signalled":

1. The file-spec given to call-with-input-file, call-with-output-file, open-input-file, or open-output-file represents a file that cannot be opened.

2. An end of file is read from a port by read after the beginning of an object's external representation, but the external representation is incomplete and therefore not parsable.

I propose that in both cases the implementation be required to raise an exception as if by applying raise (that is, non-continuably) to an implementation-defined object, which means it can be caught by the R7RS exception system. Note that there is no requirement to create a fresh object.