Improve readability of plaintext reports

[AlexVanderbist]

Currently the plain text version of the exception and stacktrace are printed in the header and footer of the Ignition response. This makes it easier to spot the exception in a CLI/test runner environment. To avoid XSS we use PHP's htmlspecialcharacters function. However, this causes some ugly HTML tags in a plain text output.

We should look into replacing some common special characters found in stacktraces with a safe equivalent. E.g. -> becomes →.