Continue after failed attempt to delete old nightly

Failure to catch this error broke bholley's PR:
http://build.servo.org/builders/linux-dev/builds/8616/steps/shell__1/logs/stdio.
Logs below for posterity.

Current Rust nightly version: 599be0d18f4c6ddf36366d2a5a2ca6dc65886896
Current Rust stable version: 1.19.0
Removing /home/servo/.servo/cargo/13d94d5fa8129a34f5c77a1bcd76983f5aed2434
Error running mach:

    ['clean-nightlies', '--keep', '3', '--force']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

OSError: [Errno 2] No such file or directory:
'/home/servo/.servo/cargo/13d94d5fa8129a34f5c77a1bcd76983f5aed2434'

  File
"/home/servo/buildbot/slave/linux-dev/build/python/servo/bootstrap_commands.py",
line 334, in clean_nightlies
    delete(full_path)
  File "/home/servo/buildbot/slave/linux-dev/build/python/servo/util.py", line
28, in delete
    os.remove(path)

python/servo/bootstrap_commands.py

@@ -331,7 +331,10 @@ class MachCommands(CommandBase):
                     full_path = path.join(base, name)
                     if force:
                         print("Removing {}".format(full_path))
-                        delete(full_path)
+                        try:
+                            delete(full_path)
+                        except OSError as e:
+                            print("Removal failed with error {}".format(e))
                     else:
                         print("Would remove {}".format(full_path))
         if not removing_anything: