FindPkgConfig: return actual error when a package is not found (#15810)

In some cases, CMake returned the following error:

-- Checking for module 'foo'
--   Package 'foo' not found

When the actual error returned by pkg-config was:

  Package 'bar', required by 'foo', not found

Now, the actual error is forwarded to the user.

-- Checking for module 'foo'
--   Package 'bar', required by 'foo', not found

For the standard case (i.e. the package was indeed not found), the
CMake error was:

-- Checking for module 'foo'
--   Package 'foo' not found

But it now prints:

-- Checking for module 'foo'
--   No package 'foo' found

The associated test was also updated. ${last} refers to the last
CLI argument.

0 files changed, 0 insertions, 0 deletions