Improve context function error descriptions

Fixes #1193.

src/context.c

@@ -607,7 +607,8 @@ GLFWAPI void glfwMakeContextCurrent(GLFWwindow* handle)
 
     if (window && window->context.client == GLFW_NO_API)
     {
-        _glfwInputError(GLFW_NO_WINDOW_CONTEXT, NULL);
+        _glfwInputError(GLFW_NO_WINDOW_CONTEXT,
+                        "Cannot make current with a window that has no OpenGL or OpenGL ES context");
         return;
     }
 
@@ -636,7 +637,8 @@ GLFWAPI void glfwSwapBuffers(GLFWwindow* handle)
 
     if (window->context.client == GLFW_NO_API)
     {
-        _glfwInputError(GLFW_NO_WINDOW_CONTEXT, NULL);
+        _glfwInputError(GLFW_NO_WINDOW_CONTEXT,
+                        "Cannot swap buffers of a window that has no OpenGL or OpenGL ES context");
         return;
     }
 
@@ -652,7 +654,8 @@ GLFWAPI void glfwSwapInterval(int interval)
     window = _glfwPlatformGetTls(&_glfw.contextSlot);
     if (!window)
     {
-        _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
+        _glfwInputError(GLFW_NO_CURRENT_CONTEXT,
+                        "Cannot set swap interval without a current OpenGL or OpenGL ES context");
         return;
     }
 
@@ -669,13 +672,14 @@ GLFWAPI int glfwExtensionSupported(const char* extension)
     window = _glfwPlatformGetTls(&_glfw.contextSlot);
     if (!window)
     {
-        _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
+        _glfwInputError(GLFW_NO_CURRENT_CONTEXT,
+                        "Cannot query extension without a current OpenGL or OpenGL ES context");
         return GLFW_FALSE;
     }
 
     if (*extension == '\0')
     {
-        _glfwInputError(GLFW_INVALID_VALUE, "Extension name is empty string");
+        _glfwInputError(GLFW_INVALID_VALUE, "Extension name cannot be an empty string");
         return GLFW_FALSE;
     }
 
@@ -734,7 +738,8 @@ GLFWAPI GLFWglproc glfwGetProcAddress(const char* procname)
     window = _glfwPlatformGetTls(&_glfw.contextSlot);
     if (!window)
     {
-        _glfwInputError(GLFW_NO_CURRENT_CONTEXT, NULL);
+        _glfwInputError(GLFW_NO_CURRENT_CONTEXT,
+                        "Cannot query entry point without a current OpenGL or OpenGL ES context");
         return NULL;
     }