Usage of the terms "parameter" and "argument"

We seem to misuse the terms "parameter" and "argument" when generate error messages. They're not synonyms and have pretty clear [definitions](https://stackoverflow.com/questions/156767/whats-the-difference-between-an-argument-and-a-parameter):
- a parameter is a variable in the declaration of a function;
- an argument is the actual value of this variable that gets passed to the function.

For instance, the code
```python
>>> from datatable import dt
>>> dt.fread(sep=1)
TypeError: Argument sep in function datatable.fread() should be a string, instead got
```

should really print
```python
TypeError: Parameter sep in function datatable.fread() should be a string, instead got
```