error logging: Simplify deployment data.

We no longer have intermediate constants of
`git_described` and `zulip_version_const`.

Instead, we make a `deployment_data` dictionary
that is grep-friendly, and we just let
`deployment_repr` do simple formatting
without translating string constants.

This is pretty easy to test:

    - set DEBUG_ERROR_REPORTING = True
    - modify some code to throw an exception
    - see error output in #errors
    - use "/emails" with text-only option to view
      errors

zerver/lib/error_notify.py

@@ -33,11 +33,8 @@ def user_info_str(report: Dict[str, Any]) -> str:
 
 def deployment_repr(report: Dict[str, Any]) -> str:
     deployment = 'Deployed code:\n'
-    for (label, field) in [('git', 'git_described'),
-                           ('ZULIP_VERSION', 'zulip_version_const'),
-                           ]:
-        if report[field] is not None:
-            deployment += '- %s: %s\n' % (label, report[field])
+    for field, val in report['deployment_data'].items():
+        deployment += '- %s: %s\n' % (field, val)
     return deployment
 
 def notify_browser_error(report: Dict[str, Any]) -> None:

zerver/logging_handlers.py

@@ -26,8 +26,10 @@ def try_git_describe() -> Optional[str]:
         return None
 
 def add_deployment_metadata(report: Dict[str, Any]) -> None:
-    report['git_described'] = try_git_describe()
-    report['zulip_version_const'] = ZULIP_VERSION
+    report['deployment_data'] = dict(
+        git=try_git_describe(),
+        ZULIP_VERSION=ZULIP_VERSION,
+    )
 
 def add_request_metadata(report: Dict[str, Any], request: HttpRequest) -> None:
     report['has_request'] = True