Address a StandardOutput Format bug; 1849:

[mmatera]

The origin of the issue was that when string_render_output_form does not get a string, fall back to the default format function. In the example, `

 mathics -c 'xx="a string"; StringTake[xx[0]]'

produced an expression with a String as the head:

In[1]:= xx="a string"
Out[1]= "a string"

In[2]:= xx[0] //FullForm
Out[2]//FullForm= "a string"[0]

When render_output_form asks for the lookup_name of the callback function, it get SystemString. But that function just processes Stringatoms, not expressions with aString` as its head. This patch just raise the exception that produces to the parent function to use the default render function.

So, now we get

StringTake::argr: StringTake called with 1 argument; 2 arguments are expected.
StringTake[a string[0]]
```

Notice that if instead of a string, the head was an integer, the result was already right:
```
(Mathics3) mauricio@mauricio-T15thinkpad:~/Projects/mathics-core$ mathics -c 'xx=7; StringTake[xx[0]]'
StringTake::argr: StringTake called with 1 argument; 2 arguments are expected.
StringTake[7[0]]
```

[rocky]

LGTM