Improve error handling

* Report errors to admin by e-mail
* Return details of error in JSON for API calls
* Show error details page for non-API calls

matcher/error_mail.py

@@ -0,0 +1,46 @@
+import logging
+from logging.handlers import SMTPHandler
+from logging import Formatter
+from flask import request
+
+PROJECT = 'osm-wikidata'
+
+class MatcherSMTPHandler(SMTPHandler):
+    def getSubject(self, record):  # noqa: N802
+        return (f'{PROJECT} error: {record.exc_info[0].__name__}'
+                if (record.exc_info and record.exc_info[0])
+                else f'{PROJECT} error: {record.pathname}:{record.lineno:d}')
+
+
+class RequestFormatter(Formatter):
+    def format(self, record):
+        record.request = request
+        return super().format(record)
+
+
+def setup_error_mail(app):
+    if not app.config.get('ERROR_MAIL'):
+        return
+    formatter = RequestFormatter('''
+    Message type:       {levelname}
+    Location:           {pathname:s}:{lineno:d}
+    Module:             {module:s}
+    Function:           {funcName:s}
+    Time:               {asctime:s}
+    GET args:           {request.args!r}
+    URL:                {request.url}
+
+    Message:
+
+    {message:s}
+    ''', style='{')
+
+    mail_handler = MatcherSMTPHandler(app.config['SMTP_HOST'],
+                                      app.config['MAIL_FROM'],
+                                      app.config['ADMINS'],
+                                      app.name + ' error')
+    mail_handler.setFormatter(formatter)
+
+    mail_handler.setLevel(logging.ERROR)
+    app.logger.propagate = True
+    app.logger.addHandler(mail_handler)