Blow up config if opentracing is missing (#5985)

* Blow up config if opentracing is missing

changelog.d/5985.feature

@@ -0,0 +1 @@
+Check at setup that opentracing is installed if it's enabled in the config.

synapse/config/tracer.py

@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from synapse.python_dependencies import DependencyException, check_requirements
+
 from ._base import Config, ConfigError
 
 
@@ -32,6 +34,11 @@ class TracerConfig(Config):
         if not self.opentracer_enabled:
             return
 
+        try:
+            check_requirements("opentracing")
+        except DependencyException as e:
+            raise ConfigError(e.message)
+
         # The tracer is enabled so sanitize the config
 
         self.opentracer_whitelist = opentracing_config.get("homeserver_whitelist", [])