userspace-probe: Print error for unsupported instrumentation mode

userspace-probe: Print error for unsupported instrumentation mode

This patch adds an error message printed when the user tries to place an
userspace-probe using unsupported probe location descriptions.
Hopefully, this will help users understand why their command failed.

Here are examples of unsupported uses of an address location:
* "elf:/path/to/binary:0x400430"
* "elf:/path/to/binary:4194364"

Here are examples of unsupported uses of offset from symbol location:
* "elf:/path/to/binary:my_symbol+0x323"
* "elf:/path/to/binary:my_symbol+43"

I expect users to try using the address locations and offset from symbol
locations for ELF instrumentation location because those methods are
available with the --probe option used to instrument the kernel.

Supporting those location descriptions in the future would require
the validation that the address or the offset from a symbol is indeed at
the boundary of an instruction.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>