Generate useful error for TA input "s"

see #15

src/main/java/edu/kit/typicalc/model/parser/TypeAssumptionParser.java

@@ -172,7 +172,9 @@ public class TypeAssumptionParser {
             if (t.getType() == TokenType.COLON) {
                 return new ParserResult<>(new ExpectingTypeDef(alreadyParsed, var));
             } else {
-                return new ParserResult<>(ParseError.unexpectedToken(t, ParseError.ErrorType.TYPE_ASSUMPTION_ERROR));
+                return new ParserResult<>(ParseError
+                        .unexpectedToken(t, ParseError.ErrorType.TYPE_ASSUMPTION_ERROR)
+                        .expectedType(TokenType.COLON));
             }
         }
     }

src/test/java/edu/kit/typicalc/model/parser/TypeAssumptionParserTest.java

@@ -349,6 +349,16 @@ class TypeAssumptionParserTest {
         assertEquals(Token.TokenType.EOF, e.getCause().get().getType());
     }
 
+    @Test
+    void errorCase3() {
+        ParseError e = parse("s");
+        assertEquals(ParseError
+                .unexpectedToken(new Token(Token.TokenType.EOF, "", "s", 1),
+                        ParseError.ErrorType.TYPE_ASSUMPTION_ERROR)
+                .expectedType(Token.TokenType.COLON),
+                e);
+    }
+
     static ParseError parse(String input) {
         return new TypeAssumptionParser().parse(input).unwrapError();
     }