Various error fixes.

Fixed incorrect error types.  Added missing error string prefixes.
Removed some invalid or superfluous error emissions.  Clarified some
error strings.  Joined error string lines to aid grep.  Replaced some
generic error strings with specific ones.  Documentation work.

Fixes #450.

src/cocoa_monitor.m

@@ -57,7 +57,8 @@ static char* getDisplayName(CGDirectDisplayID displayID)
                                                  (const void**) &value))
     {
         // This may happen if a desktop Mac is running headless
-        _glfwInputError(GLFW_PLATFORM_ERROR, "Failed to retrieve display name");
+        _glfwInputError(GLFW_PLATFORM_ERROR,
+                        "Cocoa: Failed to retrieve display name");
 
         CFRelease(info);
         return strdup("Unknown");