Allow idx 0 err to be put into error_name_table
Allow idx 0 err to be put into error_name_table

This way, in the very situation where a function has a return type
an error union such as `anyerror!void` but doesn't have any erroneous
paths, calling `@errorName` on the unpacked error (which will never
be triggered) will not trip up the static analyzer.

andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig