Truncate Exception Messages if needed.

If an exception occurs, such as a `Mysql2::Error::ConnectionError`, where the query is extremely long, this can cause the payload to exceed what Bugsnag will accept and throw a `Errno::EPIPE: Broken pipe` error when trying to send the request to Bugsnag.

A number of elements are already trimmed when the payload is too large, including meta data, stacktrace code, etc. However, the exception messages were not being trimmed.

This commit adds exception message trimming to `Bugsnag::Helpers.trim_if_needed`, which should help reduce errors when the queries causing the errors are exceptionally long.

lib/bugsnag/helpers.rb

@@ -19,8 +19,12 @@ def self.trim_if_needed(value)
 
       return value unless payload_too_long?(value)
 
+      # Truncate exception messages
+      reduced_value = truncate_exception_messages(value)
+      return reduced_value unless payload_too_long?(reduced_value)
+
       # Trim metadata
-      reduced_value = trim_metadata(value)
+      reduced_value = trim_metadata(reduced_value)
       return reduced_value unless payload_too_long?(reduced_value)
 
       # Trim code from stacktrace
@@ -71,6 +75,15 @@ def self.deep_merge!(l_hash, r_hash)
 
     TRUNCATION_INFO = '[TRUNCATED]'
 
+    ##
+    # Truncate exception messages
+    def self.truncate_exception_messages(payload)
+      extract_exception(payload) do |exception|
+        exception[:message] = trim_as_string(exception[:message])
+      end
+      payload
+    end
+
     ##
     # Remove all code from stacktraces
     def self.trim_stacktrace_code(payload)