Make error type bool conversion operators explicit

This avoids ambiguities caused by overload resolution trying to use
these conversion operators to jump to other types in implicit conversion
(i.e. QTextStream& operator<<(QTextStream& ts, const Qx::Error e), but
still allows simple statements like:

if(errorDerivedInstance)

because these explicit conversion function are still allowed to be used
somewhat implicitly in "conditional contexts".

See https://en.cppreference.com/w/cpp/language/
implicit_conversion#Contextual%20conversions

lib/core/include/qx/core/qx-abstracterror.h

@@ -105,7 +105,7 @@ friend class Error;
 public:
     bool operator==(const AbstractError& other) const = default;
     bool operator!=(const AbstractError& other) const = default;
-    operator bool() const { return deriveValue() > 0; };
+    explicit operator bool() const { return deriveValue() > 0; };
 };
 
 /* TODO: Get string of the type automatically when it becomes

lib/core/include/qx/core/qx-error.h

@@ -114,7 +114,7 @@ class QX_CORE_EXPORT Error
 public:
     bool operator==(const Error& other) const = default;
     bool operator!=(const Error& other) const = default;
-    operator bool() const;
+    explicit operator bool() const;
 
 //-Friend Functions------------------------------------------------------------------------------------------------
 friend QTextStream& ::operator<<(QTextStream& ts, const Error& e);