osd-target-run: Ensure that ELF file exists and is a file

[imphil]

Passing a non-existing ELF file, or a directory, to osd-target-run does not result in an error message, but in wrong behavior:

$ ~/swinstall/osd/bin/osd-target-run -e /home/philipp/src/diasys/casestudies/transactioncheck/bug-negative-msgs --coretrace --systrace
[ERROR] libosd: invalid file descriptor
[ERROR] osd-target-run: Unable to use ELF file /home/philipp/src/diasys/casestudies/transactioncheck/bug-negative-msgs to decode control trace.
[ERROR] libosd: invalid file descriptor
[ERROR] osd-target-run: Unable to use ELF file /home/philipp/src/diasys/casestudies/transactioncheck/bug-negative-msgs to decode control trace.
[ERROR] libosd: invalid file descriptor
[ERROR] osd-target-run: Unable to use ELF file /home/philipp/src/diasys/casestudies/transactioncheck/bug-negative-msgs to decode control trace.