Fix #283: don't print first arg of constructor

This changes the exception error message of a bad-arguments error to
suppress the constructor argument when the failure is a constructor.

This changes both the "Invoked with: " output to omit the object
instances, and rewrites the constructor signature to make it look
like a constructor (changing the first argument to the object name, and
removing the ' -> NoneType' return type.
diff --git a/example/issues.cpp b/example/issues.cpp
index 8314759..55fc3f3 100644
--- a/example/issues.cpp
+++ b/example/issues.cpp
@@ -138,4 +138,23 @@
     } catch (std::runtime_error &) {
         /* All good */
     }
+
+    // Issue #283: __str__ called on uninitialized instance when constructor arguments invalid
+    class StrIssue {
+    public:
+        StrIssue(int i) : val{i} {}
+        StrIssue() : StrIssue(-1) {}
+        int value() const { return val; }
+    private:
+        int val;
+    };
+    py::class_<StrIssue> si(m2, "StrIssue");
+    si  .def(py::init<int>())
+        .def(py::init<>())
+        .def("__str__", [](const StrIssue &si) {
+                std::cout << "StrIssue.__str__ called" << std::endl;
+                return "StrIssue[" + std::to_string(si.value()) + "]";
+                })
+        ;
+
 }
diff --git a/example/issues.py b/example/issues.py
index 257f08e..716a1b2 100644
--- a/example/issues.py
+++ b/example/issues.py
@@ -10,6 +10,7 @@
 from example.issues import ElementList, ElementA, print_element
 from example.issues import expect_float, expect_int
 from example.issues import A, call_f
+from example.issues import StrIssue
 import gc
 
 print_cchar("const char *")
@@ -72,3 +73,8 @@
 b = B()
 call_f(b)
 
+print(StrIssue(3))
+try:
+    print(StrIssue("no", "such", "constructor"))
+except TypeError as e:
+    print("Failed as expected: " + str(e))
diff --git a/example/issues.ref b/example/issues.ref
index 58cc798..0386d2c 100644
--- a/example/issues.ref
+++ b/example/issues.ref
@@ -18,3 +18,9 @@
 PyA.PyA()
 PyA.f()
 In python f()
+StrIssue.__str__ called
+StrIssue[3]
+Failed as expected: Incompatible constructor arguments. The following argument types are supported:
+    1. example.issues.StrIssue(int)
+    2. example.issues.StrIssue()
+    Invoked with: no, such, constructor