This checked specifically for "| and" and "a=b" and then just gave the
error without a caret at all.
E.g. for a /tmp/broken.fish that contains
```fish
echo foo
echo foo | and cat
```
This would print:
```
/tmp/broken.fish (line 3): The 'and' command can not be used in a pipeline
warning: Error while reading file /tmp/broken.fish
```
without any indication other than the line number as to the location
of the error.
Now we do
```
/tmp/broken.fish (line 3): The 'and' command can not be used in a pipeline
echo foo | and cat
^~^
warning: Error while reading file /tmp/broken.fish
```
Another nice one:
```
fish --no-config -c 'echo notprinted; echo foo; a=b'
```
failed to give the error message!
(Note: Is it really a "warning" if we failed to read the one file we
wer told to?)
We should check if we should either centralize these error messages
completely, or always pass them and remove this "code" system, because
it's only used in some cases.
This one tests a bunch of separate stuff, so we put it into a few
different files.
The main, new one is "slices.fish", which tests various index expressions.
