Don't abort rustdoc tests if `tidy` isn't installed

Don't abort rustdoc tests if `tidy` isn't installed

Before:

```
Check compiletest suite=rustdoc mode=rustdoc (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 396 tests
..................................................2020-11-23T12:12:37.735649Z ERROR compiletest::runtest: fatal error, panic: "failed to run tidy - is it installed? - No such file or directory (os error 2)"
F................................................. 100/396
.................................................................................................... 200/396
.................................................................................................... 300/396
...............................i...............2020-11-23T12:15:00.271271Z ERROR compiletest::runtest: fatal error, panic: "failed to run tidy - is it installed? - No such file or directory (os error 2)"
F................................................
```

After:

```
Check compiletest suite=rustdoc mode=rustdoc (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)

running 4 tests
.FFF
failures:

---- [rustdoc] rustdoc/fn-pointer-arg-name.rs stdout ----

error: htmldocck failed!
status: exit code: 1
command: "/usr/bin/python" "/home/joshua/rustc/src/etc/htmldocck.py" "/home/joshua/rustc/build/x86_64-unknown-linux-gnu/test/rustdoc/fn-pointer-arg-name" "/home/joshua/rustc/src/test/rustdoc/fn-pointer-arg-name.rs"
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
4: @has check failed
	`XPATH PATTERN` did not match
	// @has - '//*[@class="rust fn"]' 'pub fn f(callback: fn(len: usize, foo: u32))'

Encountered 1 errors

------------------------------------------

info: generating a diff against nightly rustdoc
failed to run tidy - is it installed? - Permission denied (os error 13)
failed to run tidy - is it installed? - Permission denied (os error 13)
 # a diff without running `tidy`
```