YAML parse errors are not handled properly and instead cause a crash

[johnbartholomew]

Example:

./jsonnet -e 'std.parseYaml("a: b:")'
make: Nothing to be done for 'default'.
1:6: (5B): ERROR: two colons on same line
1:6: a: b:  (size=5)
          ^  (cols 6-6)

Aborted                    (core dumped) ./jsonnet -e 'std.parseYaml("a: b:")'

RapidYAML produces an exception but it's not caught so it just kills everything. Instead the exception should be caught and an appropriate Jsonnet error with backtrace should be produced. Ideally the YAML parse error information should still be exposed through the error message.