Clean up/improve e2e logging

In an attempt to make the rageshake logs a bit more useful, try to make the
logging a bit saner. Firstly, make sure we log every decryption failure, and
log it exactly once, rather than in several places. Also record when we receive
megolm keys. Also add some more explicit logging in the sync loop.

src/crypto/algorithms/base.js

@@ -148,14 +148,42 @@ DecryptionAlgorithm.prototype.importRoomKey = function(session) {
 /**
  * Exception thrown when decryption fails
  *
+ * @alias module:crypto/algorithms/base.DecryptionError
  * @constructor
- * @param {string} msg message describing the problem
+ * @param {string} msg user-visible message describing the problem
+ *
+ * @param {Object=} details key/value pairs reported in the logs but not shown
+ *   to the user.
+ *
  * @extends Error
  */
-module.exports.DecryptionError = function(msg) {
+const DecryptionError = function(msg, details) {
+    this.name = 'DecryptionError';
     this.message = msg;
+    this.details = details;
 };
-utils.inherits(module.exports.DecryptionError, Error);
+utils.inherits(DecryptionError, Error);
+
+/** override the string used when logging
+ *
+ * @returns {String}
+ */
+DecryptionError.prototype.toString = function() {
+    let result = this.name + '[msg: '+ this.message;
+
+    if (this.details) {
+        result += ', ' +
+            Object.keys(this.details).map(
+                (k) => k + ': ' + this.details[k],
+            ).join(', ');
+    }
+
+    result += ']';
+
+    return result;
+};
+
+module.exports.DecryptionError = DecryptionError;
 
 /**
  * Exception thrown specifically when we want to warn the user to consider