Improve an error message when trying to use 'args' other than tracepoint
Improve an error message when trying to use 'args' other than tracepoint

This cahnges an error message

```
% sudo ./src/bpftrace -e 'k:f,t:syscalls:sys_enter_open {@=str(args->filename);}'
bpftrace: ../src/ast/semantic_analyser.cpp:1285: virtual void bpftrace::ast::SemanticAnalyser::visit(bpftrace::ast::FieldAccess &): Assertion `probetype(attach_point->provider) == ProbeType::tracepoint' failed.
zsh: abort      sudo ./src/bpftrace -e
```

to

```
% sudo ./src/bpftrace -e 'k:f,t:syscalls:sys_enter_open {@=str(args->filename);}'
stdin:1:1-41: ERROR: The args builtin can only be used with tracepoint probes (kprobe used here)
k:45d74427ec8709255bbd85f33cdff69bb049f828@=str(args->filename);}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace