Don't ignore invalid code related to subproject calls

We should only silently return from a dependency() call if the error is
transient (old version, wrap failed to download etc), not if the
subproject invocation or dependency name are incorrect.

For instance, if you use a dependency(..., fallback : ...) call in
a meson.build not in the root directory, we would always ignore the
call instead of erroring out due to invalid usage.

We should consider categorising our exceptions in this manner elsewhere
too.

mesonbuild/interpreter.py

@@ -1555,7 +1555,7 @@ class Interpreter():
         if dirname in self.subproject_stack:
             fullstack = self.subproject_stack + [dirname]
             incpath = ' => '.join(fullstack)
-            raise InterpreterException('Recursive include of subprojects: %s.' % incpath)
+            raise InvalidCode('Recursive include of subprojects: %s.' % incpath)
         if dirname in self.subprojects:
             return self.subprojects[dirname]
         r = wrap.Resolver(os.path.join(self.build.environment.get_source_dir(), self.subproject_dir))
@@ -1908,8 +1908,14 @@ requirements use the version keyword argument instead.''')
 
     def dependency_fallback(self, name, kwargs):
         dirname, varname = self.get_subproject_infos(kwargs)
+        # Try to execute the subproject
         try:
             self.do_subproject(dirname, {})
+        # Invalid code is always an error
+        except InvalidCode:
+            raise
+        # If the subproject execution failed in a non-fatal way, don't raise an
+        # exception; let the caller handle things.
         except:
             mlog.log('Also couldn\'t find a fallback subproject in',
                     mlog.bold(os.path.join(self.subproject_dir, dirname)),
@@ -1918,13 +1924,11 @@ requirements use the version keyword argument instead.''')
         try:
             dep = self.subprojects[dirname].get_variable_method([varname], {})
         except KeyError:
-            mlog.log('Fallback variable', mlog.bold(varname),
-                    'in the subproject', mlog.bold(dirname), 'does not exist')
-            return None
+            raise InvalidCode('Fallback variable {!r} in the subproject ' 
+                              '{!r} does not exist'.format(varname, dirname))
         if not isinstance(dep, DependencyHolder):
-            mlog.log('Fallback variable', mlog.bold(varname),
-                    'in the subproject', mlog.bold(dirname),
-                    'is not a dependency object.')
+            raise InvalidCode('Fallback variable {!r} in the subproject {!r} is '
+                              'not a dependency object.'.format(varname, dirname))
             return None
         # Check if the version of the declared dependency matches what we want
         if 'version' in kwargs: