Reduce an indent level in the install for loop

if not t.should_install(): continue

No logic changes at all.

Also improve the message when erroring out about insufficient
install_dir: list elements.

mesonbuild/backend/ninjabackend.py

@@ -628,8 +628,8 @@ int dummy;
 
     def generate_target_install(self, d):
         for t in self.build.get_targets().values():
-            if t.should_install():
-                should_strip = self.get_option_for_target('strip', t)
+            if not t.should_install():
+                continue
             # Find the installation directory.
             outdirs = t.get_custom_install_dir()
             custom_install_dir = False
@@ -651,11 +651,13 @@ int dummy;
             # Sanity-check the outputs and install_dirs
             num_outdirs, num_out = len(outdirs), len(t.get_outputs())
             if num_outdirs != 1 and num_outdirs != num_out:
-                    raise MesonException('Target {!r} has {} outputs, but only '
-                                         '{} "install_dir"s were specified'
-                                         ''.format(t.name, num_out, num_outdirs))
+                m = 'Target {!r} has {} outputs: {!r}, but only {} "install_dir"s were found.\n' \
+                    "Pass 'false' for outputs that should not be installed and 'true' for\n" \
+                    'using the default installation directory for an output.'
+                raise MesonException(m.format(t.name, num_out, t.get_outputs(), num_outdirs))
             # Install the target output(s)
             if isinstance(t, build.BuildTarget):
+                should_strip = self.get_option_for_target('strip', t)
                 # Install primary build output (library/executable/jar, etc)
                 # Done separately because of strip/aliases/rpath
                 if outdirs[0] is not False: