Fix error messages being duplicated

[alexjpwalker]

What is the goal of this PR?

Previously, when parsing a query failed with an error, on converting the resulting error object to a string, the string would contain two copies of the error message - e.g:

[GQL03] TypeQL Error: There is a syntax error at line 1:
match $x sb thing;
         ^
no viable alternative at input 'match $x sb'
[GQL03] TypeQL Error: There is a syntax error at line 1:
match $x sb thing;
         ^
no viable alternative at input 'match $x sb'

This is now fixed and error messages are no longer duplicated. (Also, the error code is now TQL)

What are the changes implemented in this PR?

• Fix error messages being duplicated (fixes Error messages are duplicated typeql#222)
• Change error code from GQL to TQL

[alexjpwalker]

These changes look good

[alexjpwalker]

The error handling logic is as follows:

1. Attach error listener to parser - whenever a parser error is thrown, it gets stored in the Listener
2. Parse query with optimised parser (returns less useful error messages)
3. If parsing fails, re-parse query with strict parser (returns more detailed error messages)
4. If parsing fails (which it will), dump all errors in the Listener to an exception string.

Trouble is, this flow causes 2 errors (which may not necessarily be identical, but often are) to be piped into the Listener, and they both make it into the output.

To fix this issue, we clean the first (less precise) error from the Listener before storing the second, more precise one.