check qemu_path correctly and give out a reasonable error msg

[Kyle-Kyle]

Should check qemu_path instead of qemu itself.

qemu will never be None so the exception will never be triggered.
But if QEMU is not installed, qemu_path will be None and later fails which(qemu_path)

[zachriggle]

The original exception should be re-raised, not the new exception message.

This error path is taken on every failure to execute (-ENOEXEC) so this ends up raising a misleading error message in the following case on e.g. macOS:

>>> from pwn import *
>>> io = process('./pwnlib/data/elf/test-aarch64')
[x] Starting local process './pwnlib/data/elf/test-aarch64'
[!] Neither 'qemu-aarch64' nor 'qemu-aarch64-static' are available
[-] Starting local process './pwnlib/data/elf/test-aarch64': Failed
...
RuntimeError: No QEMU runner available, please install QEMU first