Generalize message about fallback failure

The fallback might be not used not only because it couldn't be found, but
also because something went wrong trying to use it.

Also, update a test which relies on the specific text

mesonbuild/interpreter.py

@@ -2735,7 +2735,7 @@ root and issuing %s.
         # If the subproject execution failed in a non-fatal way, don't raise an
         # exception; let the caller handle things.
         except Exception as e:
-            mlog.log('Also couldn\'t find a fallback subproject in',
+            mlog.log('Couldn\'t use fallback subproject in',
                      mlog.bold(os.path.join(self.subproject_dir, dirname)),
                      'for the dependency', mlog.bold(name), '\nReason:', str(e))
             return None

run_unittests.py

@@ -2308,13 +2308,13 @@ class FailureTests(BasePlatformTests):
         '''
         tdir = os.path.join(self.unit_test_dir, '20 subproj dep variables')
         out = self.init(tdir, inprocess=True)
-        self.assertRegex(out, r"Also couldn't find a fallback subproject "
+        self.assertRegex(out, r"Couldn't use fallback subproject "
                          "in.*subprojects.*nosubproj.*for the dependency.*somedep")
         self.assertRegex(out, r'Dependency.*somenotfounddep.*from subproject.*'
                          'subprojects.*somesubproj.*found:.*NO')
         self.assertRegex(out, r'Dependency.*zlibproxy.*from subproject.*'
                          'subprojects.*somesubproj.*found:.*YES.*(cached)')
-        self.assertRegex(out, r'Also couldn\'t find a fallback subproject in '
+        self.assertRegex(out, r'Couldn\'t use fallback subproject in '
                          '.*subprojects.*failingsubproj.*for the dependency.*somedep')
 
     def test_exception_exit_status(self):