Unhelpful error message if passing no parameter to `read_systemverilog`

[hzeller]

If there is no parameter given to read_systemverilog, the error message is strange: it complains that a file read_systemverilog can not be read.

Observed

yosys> plugin -i systemverilog
yosys> read_systemverilog 

1. Executing Verilog with UHDM frontend.
ERROR: Can't open input file `read_systemverilog' for reading: No such file or directory

Of course read_systemverilog is not a file we want to read in the first place.

Expected

Ideally the 'help' output of the plugin, or a one liner describing how to get help.

No parameter given. Consult
  help read_systemverilog

Speaking of which: there is no read_systemverilog -help. I suspect that is by design as the yosys built-in help is usually used ?

[hzeller]

Assigning to Karol to triage.