More tests to cover missing fallback for a non-found dependency

Fix logic so we correctly raise an exception about missing fallback in the
case where the dependency is required.

mesonbuild/interpreter.py

@@ -2841,7 +2841,7 @@ external dependencies (including libraries) must go to "dependencies".''')
 
             # Unless a fallback exists and is forced ...
             if self.coredata.wrap_mode == WrapMode.forcefallback and 'fallback' in kwargs:
-                exception = DependencyException("fallback for %s not found" % name)
+                pass
             # ... search for it outside the project
             elif name != '':
                 try:
@@ -2852,6 +2852,8 @@ external dependencies (including libraries) must go to "dependencies".''')
             # Search inside the projects list
             if not dep.found():
                 if 'fallback' in kwargs:
+                    if not exception:
+                        exception = DependencyException("fallback for %s not found" % display_name)
                     fallback_dep = self.dependency_fallback(name, kwargs)
                     if fallback_dep:
                         # Never add fallback deps to self.coredata.deps since we

test cases/common/171 not-found dependency/meson.build

@@ -11,3 +11,4 @@ library('testlib', 'testlib.c', dependencies: [dep])
 subdir('sub', if_found: dep)
 
 subdep = dependency('', fallback: ['trivial', 'trivial_dep'])
+missing = dependency('', fallback: ['missing', 'missing_dep'], required: false)

test cases/failing/80 subproj dependency not-found and required/meson.build

@@ -0,0 +1,2 @@
+project('dep-test')
+missing = dependency('', fallback: ['missing', 'missing_dep'], required: true)