Auto merge of #6738 - mbrubeck:bootstrap, r=larsbergstrom

Perform argument validation before bootstrapping.

If there's an error in the command-line arguments for `mach build`, we should print it before starting a (potentially) long bootstrap process, not after.

r? @larsbergstrom or @frewsxcv

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python/servo/build_commands.py

@@ -142,8 +142,6 @@ class MachCommands(CommandBase):
                      help="Command-line arguments to be passed through to Cargo")
     def build(self, target=None, release=False, dev=False, jobs=None,
               android=None, verbose=False, debug_mozjs=False, params=None):
-        self.ensure_bootstrapped()
-
         if android is None:
             android = self.config["build"]["android"]
 
@@ -175,6 +173,8 @@ class MachCommands(CommandBase):
             print("Please specify either --dev or --release.")
             sys.exit(1)
 
+        self.ensure_bootstrapped()
+
         if release:
             opts += ["--release"]
         if target: