exercises(rna-transcription): remove untested RnaError (#258)

Clarify that the exercise does not currently require validating the
input.

Closes: #214

exercises/practice/rna-transcription/.docs/instructions.append.md

@@ -0,0 +1,6 @@
+# Instructions append
+
+## Input validation
+
+This exercise does not require you to validate the input DNA strand.
+Each tested input contains only the characters `A`, `C`, `G`, or `T`.

exercises/practice/rna-transcription/.meta/example.zig

@@ -1,22 +1,15 @@
 const std = @import("std");
 const mem = std.mem;
 
-pub const RnaError = error{
-    IllegalDnaNucleotide,
-};
-
-pub fn toRna(allocator: mem.Allocator, dna: []const u8) ![]const u8 {
+pub fn toRna(allocator: mem.Allocator, dna: []const u8) mem.Allocator.Error![]const u8 {
     var rna_slice = try allocator.alloc(u8, dna.len);
     for (dna) |dna_nucleotide, i| {
         switch (dna_nucleotide) {
             'A' => rna_slice[i] = 'U',
             'C' => rna_slice[i] = 'G',
             'G' => rna_slice[i] = 'C',
             'T' => rna_slice[i] = 'A',
-            else => {
-                allocator.free(rna_slice);
-                return RnaError.IllegalDnaNucleotide;
-            },
+            else => unreachable,
         }
     }
     return rna_slice;

exercises/practice/rna-transcription/rna_transcription.zig

@@ -1,6 +1,6 @@
 // Import the appropriate standard library and modules
 
-pub fn toRna(allocator: mem.Allocator, dna: []const u8) (RnaError || mem.Allocator.Error)![]const u8 {
+pub fn toRna(allocator: mem.Allocator, dna: []const u8) mem.Allocator.Error![]const u8 {
     _ = allocator;
     _ = dna;
     @compileError("please implement the toRna function");