Rollup merge of #71819 - jyn514:check-for-tools, r=Mark-Simulacrum

x.py: Give a more helpful error message if curl isn't installed

Before:

```
Updating only changed submodules
Submodules updated in 0.01 seconds
Traceback (most recent call last):
  File "./x.py", line 11, in <module>
    bootstrap.main()
  ...
  File "/home/joshua/src/rust/src/bootstrap/bootstrap.py", line 137, in run
    ret = subprocess.Popen(args, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 394, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1047, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory
```

After:

```
Updating only changed submodules
Submodules updated in 0.01 seconds

spurious failure, trying again

spurious failure, trying again

spurious failure, trying again

spurious failure, trying again
failed to run: curl -s -y 30 -Y 10 --connect-timeout 30 --retry 3 -Sf -o /tmp/tmpSWF21P.sha256 https://static.rust-lang.org/dist/2020-04-22/rust-std-beta-x86_64-unknown-linux-gnu.tar.gz.sha256: [Errno 2] No such file or directory
Build completed unsuccessfully in 0:00:00
```

0 files changed, 0 insertions, 0 deletions