isImplicitlyConvertibleTo(): Add better error messages

test/libsolidity/syntaxTests/inheritance/wrong_type_base_arguments.sol

@@ -6,4 +6,4 @@ contract Derived2 is Base {
   constructor() Base(2) public { }
 }
 // ----
-// TypeError: (74-77): Invalid type for argument in constructor call. Invalid implicit conversion from int_const 300 to uint8 requested.
+// TypeError: (74-77): Invalid type for argument in constructor call. Invalid implicit conversion from int_const 300 to uint8 requested. Literal is too large to fit in uint8

test/libsolidity/syntaxTests/nameAndTypeResolution/315_fixed_type_invalid_implicit_conversion_size.sol

@@ -5,4 +5,4 @@ contract test {
     }
 }
 // ----
-// TypeError: (75-92): Type ufixed128x18 is not implicitly convertible to expected type ufixed248x8.
+// TypeError: (75-92): Type ufixed128x18 is not implicitly convertible to expected type ufixed248x8. Too many fractional digits.

test/libsolidity/syntaxTests/types/rational_number_signed_to_unsigned.sol

@@ -0,0 +1,7 @@
+contract c {
+    function f() public pure {
+        uint a = -1;
+    }
+}
+// ----
+// TypeError: (52-63): Type int_const -1 is not implicitly convertible to expected type uint256. Cannot implicitly convert signed literal to unsigned type.

test/libsolidity/syntaxTests/types/rational_number_too_large.sol

@@ -0,0 +1,7 @@
+contract c {
+    function f() public pure {
+        uint8 a = 256;
+    }
+}
+// ----
+// TypeError: (52-65): Type int_const 256 is not implicitly convertible to expected type uint8. Literal is too large to fit in uint8