fix: PGPool error handling and app.js catch/callback bugs

[arnoldcastro5000]

What

Fixes four confirmed bugs in PGPool.js and app.js that cause
permanent request freezes, memory leaks, unhandled process crashes,
and broken 500 error responses.

Fixes Bug 1 in issue: Greenstand/treetracker-infrastructure#315
Related to issue: Greenstand/treetracker-infrastructure#300

Why

PGPool.js

• A DB error left isFetching[sql] = true permanently, freezing all
  future requests for that SQL string and leaking queued callbacks
• The getQuery() Promise never rejected on DB error — callers hung
  indefinitely with no error signal

app.js

• All 4 route catch blocks referenced undefined error instead of
  caught e, causing a ReferenceError that prevented 500 responses
  from being sent
• throw err inside non-async map.render() and fromString
  callbacks bypassed the outer try/catch, generating unhandled
  exceptions that crash the Node process

Change

PGPool.js

• Adopted Node.js error-first callback convention cb(err, result)
  on both success and error paths
• catch block now resets isFetching[sql] and drains the queue,
  calling each pending callback with the error
• getQuery() consumer updated to handle (err, result) and
  propagate errors via rej(err)

app.js

• fromString callback: replaced throw "failed" with return rej(err)
• Both grid routes: renamed _rej → rej to bring rejector into scope
• All render callbacks: replaced throw err with return rej(err)
• All 4 catch blocks: fixed error → e

Follow-up

• Tests for new error paths in PGPool.js (fetch failure, queue drain)
  and app.js (rej propagation) to be added in a subsequent PR per
  CONTRIBUTING.md testing requirements.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

[arnoldcastro5000]

@dadiorchen for your review.

Fixes Bug 1 in issue: Greenstand/treetracker-infrastructure#315

[arnoldcastro5000]

The 2 stuck pods need to be cleared separately. They are not handled by this PR.

PS D:\git\node-mapnik-1> kubectl get pods -n tile-server
NAME READY STATUS RESTARTS AGE
treetracker-tile-server-76c56645c4-7kbfj 1/1 Running 11 (7h3m ago) 22d
treetracker-tile-server-76c56645c4-cn2pt 0/1 ContainerStatusUnknown 6 (39d ago) 41d
treetracker-tile-server-76c56645c4-slzls 1/1 Running 28 (7h3m ago) 36d
treetracker-tile-server-76c56645c4-z8k82 0/1 ContainerStatusUnknown 6 (39d ago) 41d
PS D:\git\node-mapnik-1>