More detailed error logging for nginx plugin (#6175)

This makes errors more useful when Nginx can't be found or when Nginx's
configuration can't be found.  Previously, a generic
`NoInstallationError` isn't descriptive enough to explain _what_ wasn't
installed or what failed without going digging into the source code.

certbot-nginx/certbot_nginx/configurator.py

@@ -136,7 +136,9 @@ def prepare(self):
         """
         # Verify Nginx is installed
         if not util.exe_exists(self.conf('ctl')):
-            raise errors.NoInstallationError
+            raise errors.NoInstallationError(
+                "Could not find a usable 'nginx' binary. Ensure nginx exists, "
+                "the binary is executable, and your PATH is set correctly.")
 
         # Make sure configuration is valid
         self.config_test()

certbot-nginx/certbot_nginx/parser.py

@@ -222,7 +222,7 @@ def _find_config_root(self):
                 return os.path.join(self.root, name)
 
         raise errors.NoInstallationError(
-            "Could not find configuration root")
+            "Could not find Nginx root configuration file (nginx.conf)")
 
     def filedump(self, ext='tmp', lazy=True):
         """Dumps parsed configurations into files.