Fail without segfault for missing bounds file

FP_SPECIAL and RESCALE_INEXACT modes now fail with reasonable error
messages when no bounds file is provided. This follows the existing
behaviour of DOT_PRODUCT and ABS_ERROR.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ib5133bd6b5c1f7994d7da1f7d41f71065aa0f0d3