Ensure syntax error messages are capitalized

**Feature**

When there is a syntax error, the error message has often a lower-case initial letter:
```
$ mypy -c 'x y'
<string>:1: error: invalid syntax  [syntax]
Found 1 error in 1 file (errors prevented further checking)
```
Capitalize these error messages for consistency.

**Pitch**

Almost all other error messages are capitalized, so the current situation is inconsistent.
