Log a warning if parsing links fails

When we fail to parse a link, like [method@PangoFontDescription.copy],
at least produce a warning with the full link text. It would be better
to have file and line information, but this is at least greppable.
2 jobs for parse-error in 1 minute and 39 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Check
failed #1154899
flake8

00:01:30

failed #1154900
allowed to fail
mypy

00:01:39

 
Name Stage Failure
failed
flake8 Check
  python3-pyflakes-2.1.1-2.fc31.noarch  python3-setuptools-41.6.0-1.fc31.noarch

Complete!
$ flake8 --ignore E501,E402,F405,W503 --show-source gidocgen
gidocgen/utils.py:181:82: E703 statement ends with a semicolon
log.warning(f"Can't parse '[{self._fragment}@{self._endpoint}]'");
^
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
mypy Check
gidocgen/gir/parser.py:791: error: Argument "gtype" to "Boxed" has incompatible type "Optional[GType]"; expected "GType"
gidocgen/gir/parser.py:809: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:851: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:873: error: Argument "name" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:873: error: Argument "symbol_prefix" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:873: error: Argument "ctype" to "Union" has incompatible type "Optional[str]"; expected "str"
Found 105 errors in 3 files (checked 13 source files)
Cleaning up file based variables
ERROR: Job failed: exit code 1