abbr: improve error handling when --erase called with no args

[ammgws]

Before

> abbr --erase
set: Erase needs a variable name

/usr/share/fish/functions/abbr.fish (line 116): 
    set -e $abbr_var_names
    ^
in function '__fish_abbr_erase'
	called on line 42 of file /usr/share/fish/functions/abbr.fish
in function 'abbr' with arguments '--erase'

(Type 'help set' for related documentation)
[2]>

After

> abbr --erase
[1]>

[ammgws]

# If run with no options, treat it like --add if we have arguments, or
# --show if we do not have any arguments.

Also this comment doesn't reflect the actual behaviour:
abbr --add (option with no arguments) ends up being treated as --show despite the option being supplied.

[ridiculousfish]

Thank you!