Better error reporting

[kubkon]

Our error reporting capabilities are restricted to a bare Zig error. For example, the following malformed YAML

key1:
  key1_1: value1_1:
  key1_2: value1_2
key2: value2
key3:
  key3_1: value3_1
  key3_2: value3_2
  key3_3: value3_3

generates the following error and an error trace (in debug and release safety build modes)

error: MalformedYaml                                                                                                                                                                                                                                                                                                           
/home/kubkon/dev/zig-yaml/src/parse.zig:475:21: 0x26dce8 in map (yaml)                                                                                                                                                                                                                                                         
                    return error.MalformedYaml;                                                                                                                                                                                                                                                                                
                    ^                                                                                                                                                                                                                                                                                                          
/home/kubkon/dev/zig-yaml/src/parse.zig:339:17: 0x2463bb in value (yaml)                                                                                                                                                                                                                                                       
                return self.map();
                ^
/home/kubkon/dev/zig-yaml/src/parse.zig:467:25: 0x26da5f in map (yaml)
            const val = try self.value();
                        ^
/home/kubkon/dev/zig-yaml/src/parse.zig:339:17: 0x2463bb in value (yaml)
                return self.map();
                ^
/home/kubkon/dev/zig-yaml/src/parse.zig:467:25: 0x26da5f in map (yaml)
            const val = try self.value();
                        ^
/home/kubkon/dev/zig-yaml/src/parse.zig:339:17: 0x2463bb in value (yaml)
                return self.map();
                ^
/home/kubkon/dev/zig-yaml/src/parse.zig:383:22: 0x23bd90 in doc (yaml)
        node.value = try self.value();
                     ^
/home/kubkon/dev/zig-yaml/src/parse.zig:313:33: 0x23a06e in parse (yaml)
                    const doc = try parser.doc();
                                ^
/home/kubkon/dev/zig-yaml/src/yaml.zig:229:9: 0x23c634 in load (yaml)
        try tree.parse(source);
        ^
/home/kubkon/dev/zig-yaml/examples/yaml.zig:95:18: 0x23f791 in main (yaml)
    var parsed = try yaml.Yaml.load(allocator, source);

The library is in dire need of some actual error reporting mechanism that would bubble up any errors and produce a printout where such an error occurred in the input source stream, etc.