add more compile-time diagnostics, implement fixes for them

Before, a lot of common errors went unnoticed, due to insufficient
compiler diagnostics. This commit fixes this by adding a lot of new
diagnostics, and fixing the issues found by this.

src/TSentry.cpp

@@ -6,7 +6,7 @@
 #include <sstream>
 
 TSentry::TSentry() {
-    if (std::strlen(S_DSN) == 0) {
+    if (std::strlen(S_DSN) == /* DISABLES CODE */ (0)) {
         mValid = false;
     } else {
         mValid = true;